There is Thor which I think is in Bodmin, dont know much about them. Greenlane in Exeter has closed and always seemed to create some comment when mentioned. ?..
St Blazey Motorcycles had a fair number of used enduro bikes in the show room when I was last there. I have bought a bike from them and found them very good to deal with
TBH you are probably better off keeping an eye on this forum "For Sales" as there is a regular turnover of dirtbikes of all shapes and sizes - plus they are usually local and I would like to think that a member would be totally honest about the bike they are selling so as to keep their reputation on the forum