Both these and the larvae in the previous observation look a better match for Croesus sp than arge to me. There are 4 species in the uk 2 of which feed on birch (according to bioimages.org.uk) - Croesus septentrionalis and latipes (think the former is more common). I have found pictures of croesus latipes larvae which match these ones, but am not sure if this is an early stage of Croesus larvae in general or a colour pattern specific to latipes. It is hard to find info on sawflies...

You may well be right - I couldn't find anything much about the early stages, and after posting, did wonder about Croesus for the later-stage larvae, but thought it was better to leave it as was rather than add a second ID,

Dave