Hi Tony,
I went there last year to take photographs of various AFV's for a modelling magazine. Preston Isaac is the owner of the collection, and its arranged in two large hangers. I'd say there are about 30-40 tanks, artilery pieces and other vehicles in the collection, most of which are in good order and WW2 dated. The collection is extremely tightly packed together, which with the low light, unfortunately pretty much scuppered my photos in terms of being published.
In addition to the vehicles, there are plenty of other militaria items that will definitely interest most people. Since it was out of his "open" season, I was also able, with Prestons permission, to climb inside some of the tanks and take pictures. T-54, T-34/85 etc..
Cobbaton Combat museum is defnitely worth a visit. I think the entrance was 10. Probably 4 hrs to look at everything thoroughly.
Rich