Sidewards slide on the saddle.
 

So I recently started up doing longer rides again last year, and one thing I started to notice was that I am sliding off the saddle sideways, I'm not literally sliding off, but my left sit bone is essentially hovering to the left side of the saddle, so this seems to really effect my ability to be comfy on the bike and get the power, any strong wind blowing a certain way or bump can easily make it hard for me to keep the bike in a straight line because the way I imagine it is because I'm not putting any force on my left sit bone, if something caused the bike to nudge right, the bike will swerve as my sit bone isn't in the way.

I've also realised I'm more comfortable on my commuter bike, especially whilst maintaining high efforts, and that bike usually has two panniers heavily weighted, interesting having a pannier only on the left side with a few D locks in increased my stability as I imagine it's pulling the bike sideways so my left sit bone is actually in contact with the saddle.

I've had numerous bike fits and they've all failed to look at this or correct it even when I've specifically mentioned it, to the point I feel I'm done with bike fits because it's A LOT of money thrown away, I need to try and troubleshoot it myself now.

It's very tiring for me this problem as it's effecting my ability to go more than 25-30 miles and I used to be doing 100+ miles but that was 5+ years ago, It just makes me upset that I'm loosing lots of cycling time and spending lots more time I could be cycling messing with the bike and I feel I'm getting closer and closer to giving any cycling apart from commuting up, but I don't want to do this, I see the hills in the distance and aspire to be munching the miles tearing it up in the hills again like I used to, but I'm really upset how bad my form is getting due to all this disruption and trying to figure how to fix it.

Honestly, I can waffle and waffle forever about this I'm that frustrated, literally half of me wants to quit but the other half wants to get those long miles in because the achievements are so worthwhile and the fun that it is but I feel alone and like I can't get any help with it, and money is an issue too so I can't be spending all the time.

I've just bought some 20mm pedal axle extenders to try as when I look at my knees I can see on the left side my knee is pointing in towards the top tube, the right knee pointing out, my stance standing is duck like, feet pointing out, cleats adjusted the same trying to mimic this.

Re the saddle I am using it's a Specialized Bridge Sport, every single saddle I've tried has the same issue, I've stuck with this saddle as in general it's the most comfy.

I'm just wildly guessing, which probably is what I do most of the time...

One leg slightly shorter or cleat position off slightly on one side? Maybe the saddle height is a tad too high. I'd try lowering it maybe 4 to 6 mm at a time (1/8th to 1/4 inch) and see if that helps.

Measure to some point on it so you know where it started out. If you don't think anything better is happening by the time you made several adjustments, then probably that's the wrong thing to be doing. Put it back where it was.

https://cimg6.ibsrv.net/gimg/bikefor...f1b4b0977c.jpg
https://cimg7.ibsrv.net/gimg/bikefor...8681ba3db1.jpg

Sorry those are the only ones I had from a video, I've done this in my phone, hopefully they're big enough for you to see.
I don't seem to be rocking side to side, just literally always offset to the left and as such left sitbone hovering.

https://cimg5.ibsrv.net/gimg/bikefor...d22f3b7e4b.png

I'll try do this with my left side, I've tried to use some software to work out angles, hopefully the points are in the correct plane.

Oh here is the other side, tried my best to check angles.

I think the bike fit guy said I have "functional leg length" and didn't recommend shims or anything.

https://cimg1.ibsrv.net/gimg/bikefor...e72de264a8.png

Maybe you just need a wider saddle. Saddles do come in different widths because we aren't all the same shape and size.
Based on the pictures I would be inclined to raise your seat height as there is too much bend in your knees at both the top and bottom of the stroke for my liking.

I used to find more wear on bike shorts on the starboard side, just below the crotch. Then I noticed that one leg tended to flop inward almost brushing the top tube. I've made an effort to maintain knees the same distance from the top tube, with a bit of help from a cleat on the shoe on the same side. Over time the asymmetrical wear pattern has disappeared.

I would say you have some sort of asymmetry that needs to be dealt with. Perhaps one keg is very much stronger than the other. Good luck with this.

Forget the numbers and angles. If you are dropping off one side, your saddle height is most likely too high for you, taking into account your own flexibility. Drop it slightly.

Read these: https://www.stevehoggbikefitting.com...ard-can-it-be/

https://www.stevehoggbikefitting.com...d-can-it-be-2/

I definitely favour my right side it gets the most power out.

I'm worried about lowering the saddle because any lower and I get horrible knee pain and my knees feel like they're crumbling when it comes to hills.

Could pedal axle extenders help in anyway by changing my stance ? I often wonder if my stance is too narrow given my heels bash the frame and chainset

If your knees feel like crumbling when on a hill, then you are probably in too high a gear ratio. Shift !

Sure you might slow down, but as you get better your speed will pick back up.

I wouldn't attribute it to that because my power output wasn't something I can't maintain (using a power meter) and I was in 34 / 28 on a 10% gradient.

It's so hard because some say raise it (but then lower saddle why would that cause tipping off one side as opposed to a higher saddle?) And some say lower it, but the knees defenitly don't like any lower it seems.

Not a question of whether you have the power to push that big a gear.

It's a question of whether your knees can handle that much torque going into the cranks.

In what way do your knees not like a lower gear?

Perhaps you have several issues going on and cleat position needs something too.

If my saddle is already at the right height based on the general methods like heel on pedal as my heel easily gets on the pedal. How do I know how much power to go?

If I'm lowering it so left leg is at right height right leg would be too low? Should I get wedges under the left?

You left off part of the instructions for that general method.

After setting it to that position, you ride it then adjust the saddle up or down a few times till you find what works.

None of the "general rules" are intended for you to think that is the exact position you must stay at. They are only to get you in the ball park.

Do you think lowering my saddle by 5mm will fix this?

I'm worn out all the videos and web pages I've been reading, I'd give up cycling if I wouldn't miss it so much.

Lowering your saddle will not help as I suspect that its already too low.
There are lots of things going on with your fit and just adjusting the saddle height is not going to be the magic fix.
My view is that your going to need to carefully isolate issues one at a time.

If you just gently ride around in a low gear without much power, do you sit comfortably and balanced on the saddle?
Is the saddle balance problem always there when riding gently or is it just an issue when you're applying power to the cranks?
Maybe you need a wider saddle.

I think that your cranks are too long for you based on your leg angles in the pictures but this may be exacerbated by your seat height being too low.
I want to see the saddle go up and see if this opens up your leg angles. Low saddles are only OK for gently riding around the neighbourhood. Raising the saddle will take some stress off your knees and give you more power. You can go too high with the saddle of course and start rocking yet you look too low to me.

I've on 170mm cranks, my Kineses is on 172.5mm

Now I've managed to acquire a Wahoo Kickr Snap as I'm moving into a house so I'll be able to use it now.

How about I take videos from every angle and upload them? Might be able to do it tonight, I was going to try earlier but my workplace has shackled me back to the desk on the phones.

​​​​​​And I'll try raising the saddle on the turbo too.

I find I'm unstable riding steady.

If it's the case because it seems to low for my right leg and as such I'm pushing myself to the side, why would it feel like my saddle is too high for my left leg?

Also my left knee is pointing towards the top tube, closer to it and my right knee is pointing outwards slightly, but away from the top tube.

OK, if your unstable on the saddle when taking it easy then the saddle has to be considered to be one of the culprits. I think that you have multiple fit issues going on yet getting yourself a new/better saddle has to be on the priority list.

In 2000 I broke by tib and fib in right leg when i was about 11

And deffo raise the saddle? Just seeing "saddle too high" and "too low" in the same set of comments confused me.

Just FYI on the cranks, I use 165 mm and I have a 34.5" inseam. Try everything else though. But if you get no luck, then as another seemed to hint, shorter cranks might help you.