A Sawfly larva I think but I don't have any literature to get me to species level.
No interactions present.
I agree with charlieb on this one, but it would be useful to know the foodplant as well as another check, if the larva is following the rules it should be on Alder.
Eriocampa ovata is one of the species included in a useful downloadable pdf on distinctive sawfly larvae, produced by John Grearson and avaiable via the Wiltshire & Swindon Biological Records Centre:
Entomologist and biological recorder
Lat/Lng: 54.9, -1.8
OS grid ref: NZ1760