3 pairs of true legs, 4 pairs of prolegs - it is a lepidopteran caterpillar.
There are plenty of diagrams and texts that explain the terminology, eg.
http://www.insectsexplained.com/caterpillars.pdf (though that's all todo with moth caterpillars)
This gives good guiance on differentiating the larvae of sawflies from moths & butterflies.
It does look familiar, but I can't bring a name to mind just now...
Yes - move it to a separate box for safety.
I think he's trying to remind me of a transparent burnet moth caterpillar,
but I don't think that's a good enough match.
Also, you'll note from this page its distribution, which may well not include where you are.
Thanks so much for your help. We found him in Cheshire in a flower pot with seedlings so his diet is a mystery. He seems to like the same leaves as the saw flies though which is interesting.
That link is v interesting and what a stunning insect. He does seem to have the black spots and he is slightly hairy so hopefully we will be lucky enough to see his transformation.
I think I know what he is! He looks like one of these guys but ofc I'm no expert so what do you think Mike?
Hmmm, I think not.
AFAIK, angle shades larvae don't have that row of black dots all the way along their sides.
When IDing any larva, there's the complication that the appearance changes as it develops (from instar to instar). Some species may also be polymorphic, eg. having a green form and a brown form .
There are lots of basically green larvae. Unless somebody else actually does recognize it, we may have to rely on you rearing it to adulthood in order to find out.
Ahh I see. Wow, entomology does fascinate me. Well Bill had some soil replenished today and he dived right in, so not sure what he is doing now. Hopefully we will see soon. :) My children are having a wonderful time caring for him.