Thanks, that's obviously what I wanted to hear - it's going to speed my re-education if I don't have to correct these things first. That video was interesting, I hadn't seen anybody demonstrating guitar like that; most seem to have a good amount of thumb showing at the top, only occasionally shofting it down more centrally like that.

Re. sitting down, I've tried with a strap. It helps (as the video in fundamentals 1 suggests, it stops me having to grip the guitar to keep it from slipping away), but I still can't get comfortable. If I rest on my leg, the curvature drags it to the right. If I shorten the strap enough that it's not resting on my leg at all, it's too high!