Thanks for feedback but no, I'm afraid it doesn't shed any more light on the matter. The camouflaged JP on the above link is a TWU machine from about ten years later when a handful of Mk.4 aircraft were camouflaged (but with red/blue roundels).
As KK says, you would think that if the aircraft had flown at Cranwell in camouflage, a photo would have emerged by now. But then, it seems that no photos of the aircraft at Thorney Island have appeared - until now.
If no M-number was applied, that makes the aircraft even more odd!