Hey, thanks for the replies.
I've experimented a bit more with some new saddles and concluded that the 143mm was definitely too wide. I can't actually physically pedal when I'm sitting right up on the widest part. My sit bones measure about 90mm which seems to be narrower than most; my position is also fairly aggressive and I have good pelvic rotation so this narrows the measurement further.
I tried a different design of saddle which is 139mm and although much better; I was still a little forward.
I also tried another design at 134mm where the 'wings' of the saddle drop quite steeply down each side; this felt much more sensible and I could really feel my sit-bones right on the most padded section at the back.
I now have a slightly flatter model that is 134mm which so far feels perfect. I think part of the issue is that a lot of the noseless saddles that I like tend to change transition from the narrow nose to the wide rear very quickly (Specialized Power, Fabric S-Line, Pro Stealth). Whereas, the one I'm using now is narrower for the front two thirds and only widens at the rear where you need it; this also means my thighs aren't rubbing anything. So anyway; so far so good!
I'm sure for most people this isn't an issue as given that most saddles tend to be in the 143mm region I have to assume that most have wider sit bones than me.