Confidence: I'm as sure as I can be.
Notes: Many of these larvae seen in recent weeks on hazel, feeding on it as in 2nd pic. There is a picture of the larva on page 153 of WTL 2009. The laravl foodplants given inc birch, alder and sallow but no mention of hazel. This one is very close to if not a final instar at 27mm, a good match for the pics in ukleps.