I don't think the sideways slip is anything you want to ignore. That ought to be addressed. How I don't know other than get with a good fitter if you can go the more expensive up front route.
I read
Steve Hogg's site quite a bit when trying to address my own issues. He trains people in his fit methods, and it might be worth finding one near you to take a look at you. They show a person listed for the UK if that's your area.
If you are going to persist here, then tell us again what your inseam is and what your saddle height is from top of the saddle where you sit to the top of the pedal from it's furthest position away from the saddle.