I agree it does kinda resemble the sentinel, which was our 1st thoughts, but there are so many differences in so many ways that I don't think it can be. It's massive! Having gone around our machine measuring etc and then looking at the sentinel plans it's quick to see that the similarities really are slim.
The problem is that I can't get any definitive answer from Douglas. One minute it's one thing and the next it's another. Rumour has it it is one of 12 machines that were deployed to Diversion airfields across the uk (at the time 12 diversion airfields were in use) but this went cold. Since that chat with them they haven't suggested anything other than "we think it might be related to the P111, but it might not. That's all we can help with, sorry"
I've managed to get a pic of the stamp plate, hopefully this might unlock another door somewhere. Also a pic of the front drivers side, I'll get a full frontal shot in the next couple days. Curiously it's also been painted green not the usual blue/grey RAF colours