After pestering my tutor he finally gave in and we've been looking at improving my picking technique.
He gave me a couple of exercises to do - a couple of repeating patterns over a couple of strings and a major scale pattern that covers 5 strings, all 16th note based - which I've been practising regularly since he gave them to me a few weeks ago. At first I was noticing a slight improvement and could do 5 repetitions of the major scale exercise cleanly at 116bpm and 2 or 3 repetitions at 120bpm before things get messy (the two string exercises were slightly faster).
However the last couple of practise sessions I've put in I can barely get through 5 repetitions at 104bpm. I've not been practising through pain and I haven't (to the best of my knowledge at least) changed my playing position. Everything just seems to fall apart when I push the metronome any faster.
I dare say that my picking below this speed is cleaner than it was but I'm finding the decrease in the top speed a bit disheartening.
My tutor is on holiday at the moment so I'm deferring to you lot for encouragement and (hopefully) and explanation of what's happening