Insect on alder

[bigdave60dog]

Hi everyone, Can you identify this insect - larva?-please.
It is velvety green about 7.5mm long and 2mm wide. It lives on alder leaves -exclusively as far as I can tell.. It lies flat (very flat) along the leaf rib during the daytime and makes small "shot holes on the leaf during dark hours (which make it easy to find)
It is segmented like a caterpillar with no real distinguishing marks. It is difficult to tell head from tail from above and you cannot see any legs or tell head from tail from this angle. (A bit like some hairstreak butterfly larvae)
It is not easily detached from it`s leaf but once detatched does not roll up but straightens to lie flat and straight as soon as it is on a leaf. It can crawl in the sam way as caterpillar. It has 6 small front legs and some hind claspers. I did not have a hand lens so cannot say how many hind claspers it has.
I am an experienced amateur entomologist but do not remeber seeing these before.
Ant Ideas please?

[charlieb]

Platycampus luridiventris?

[bigdave60dog]

Thanks. Yeah you got it. That`s the one. Maybe a first for Orkney which is whee we are.