As Derek suggested the larva is likely to be Brimstone moth. I was not 100% sure as I have never seen this species in the larval form before. I sent your image to Reg Fry and he comfirms it is indeed likely to be Brimstone. Try to rear through if you can and then Andy will have confirmed breeding at CB. I have always assumed it did breed at CB but its nice to have confirmation.
Blwyddyn Newydd Dda from Sue and Paul