www.ukbutterflies.co.uk

I'm desperate to know what caterpillar this is - any ideas?

Thanks

I think you will find this is not the caterpillar of a butterfly or moth... It looks like a sawfly larvae of some sort to me.
What plant was it on?... If you can find it again, count the velcro-like legs on each side. Caterpillars have 5 (pairs) but although it is not possible to tell from your photo, I suspect it will have more.... like a sawfly

Trev

Thanks Trev,
I didn't know to count the legs
I've googled it and found it to be an Alder sawfly, which is actually a bit disappointing. At least I can put a name to it now - thanks.
Alison

Many people (me included) have discovered one of these strikingly-coloured larvae in the past and wondered what kind of butterfly or moth it turns into, so you are in very good company . The reason I suspected it was a sawfly of some sort is the way it appeared to be hanging on to the leaf with it's front (proper) legs and curling the other half of its body over into a circle - very characteristic behaviour for sawfly larvae. Glad you found out the exact species Alison - now we will all know next time

Trev