Thanks for the advice, kerioboe, maya and sbhoa. It's good to know that I'm normal! In a good session I can keep going to 10 minutes, which is an improvement on when I started. Practising in short bursts suits well anyway, as that's pretty much how I manage to fit it in.
I've tried the metronome tip already this evening. I like anything that allows me to measure progress!
Maya: I
think I'm getting on well. My teacher is a jazz player and we're doing lots of work on just playing by ear and a little bit of improvising (worked on Baa, Baa Black Sheep last week). It's a completely new way of working for me; I've always been a reader and not at all a creative player, although I have a fairly good ear as long as it's just one line. He's not a very 'by the book' kind of teacher but he does have lots of teaching experience and VAST playing experience so it's all very new and exciting and inspiring. I find jazz harmony absolutely bewildering, though, and it's frustrating to be able to hear in your head what you want to play but not be able to make nice sounds or find the right notes quickly enough.
Right, I"m off to do another 10 minutes.