Ian, you could try www.matadormodels.co.uk
I know they have a David Brown in 1/72nd, I have the advert in SAMI in front of me. This is pretty much the one that was in the Airfix Stirling kit. But they also do 1/48th stuff.
There is a real one not too far from you at Newark Aircraft museum. I was there in June. It's in a brick building near to the entrance building, along with a Folland Gnat that's being rebuilt.
We used to have a 1943 David Brown at RAF Odiham until about 1979. It was used by the servicing hangar (ASF) to tow Wessex & Puma's about (so there's a good diorama!) It was a pig to drive with it's 'crash' gearbox! Then someone did crash it (not me) & we never saw it again, shame!
Hope that helps
Pete