Which British butterfly lays its eggs on Willow trees & has large (50 -60mm) pale green caterpillars?

Question

that match the under side of the Willow leaves perfectly and even have small harmless spikes on their tails to match the stalks of the surrounding leaves?

Answer 1

I am not sure but these links are handy so give them a try and even if you do not find what you are looking for you will be able to ID others in the future -

http://www.uksafari.com/caterpillars.htm

http://www.whatsthiscaterpillar.co.uk/am...

http://www.butterfliesandmoths.org/