Thanks for your help Chris.
I've done some Googling on sawfly larva and think your suggestion is quite a strong possibility. The main defining chartacteristic (of sawfly larva) seems to be the presence of six or more leg pairs on the abdominal segments - apparently caterpillars never have more than five pairs.
Frustratingly I did take some shots that better showed the legs but as they were quite blurred I deleted them all
. Lesson learned to keep all pics that might help with identification!
Bruce