Green caterpillar

I have spent the day working in the garden and doing some piano practice for a gig on Saturday.

I found this bright green 4cm long caterpillar in the garden but am not sure about it's id as there are a lot of green caterpillars and none that looked exactly like this. If anyone knowswhat it is, I would be glad to know.

I suspect that it might have started to pupate as it hasn't moved all day.  It's on the stem of a Welsh poppy and quite visible, I hope that the birds won't find it!