These are leaf beetle larvae (family Chrysomelidae). See another thread here: http://www.nhm.ac.uk/natureplus/message/30143#30143
Thanks for that speedy response. I've just been looking these up on the web and there are quite a few different species within that family aren't there. can you aim me at a website that can narrow this down a bit? I know that the larvae are black and feed on Willow so I should be able to eliminate quite a few.
I'm sorry, but I don't know such a resource. Identifying beetles from photos can be difficult, but identifying larvae is even more so. There are no books on larvae as far as I know. Read this expert's blog to get an idea of how easy would be to ID these bugs: