Thanks for all the ideas folks.
I pit roasted it in the end - after takingadvice from several people at the meet everyone had a different idea about how long it would take, so I settled on 3 hours and a bit.
Stones were heated for about 3 hours until red hot, then a large flat one placed in the pit, the venison (wrapped in foil - sorry all you purists) was placed on the stone and surrounded with the other stones. Potatoes were put in at the same time.
Once covered there was a gap of about 3 inches to the top of the pit - this ( I was reliably informed) would enhance the steaming effect.
The pit was then topped with small batons, and covered with spagnum moss. The whole lot was then covered over with the excavated earth and left.
Venison turnred out perfectly - i don't think its possible to overcook the meat using this method as the temperature drops away, slowing the cooking process.
The whole thing was served up with a crab apple and blackberry sauce - thanks to Lewis and Lloyd - and it dissapeared so fast I had to count my fingers at the end.