Thanks very much for your replies.

I can reach forwards and touch my toes without any problem at all, and my side flexion is reasonably good too. The movement I always have problems with is arching backwards - my thoracic spine moves but my lumbar spine just doesn't - what I tend to do is push my hips forwards and arch that way. I hadn't realised I was doing this until my physio held my hips still and I found I just COULDN'T arch backwards!

So it does look like I need to get some movement in my lumbar spine, but I'm just not sure that the extension exercise I've been doing is the way to do it because it doesn't seem to get to the problematic joints....