I did mine myself, took the old open fire out bought the flue and connectors on internet for a good price, got the stove from a local showroom who price matched (they thought I was going to buy the flue as well I think). I fitted it with my next door neighbour in one day with a hired roof ladder. Easy to do and I got the building inspector round and got building regs on it. The only other cost was some beer and the cost of the meal we cooked our neighbours!
The roof parts a bit jiggery pokery taking the chimney off, but I re cemented the ridge tiles down while I was up there, a nice job to do all-round.
As far as I’m aware you defo do need regs or HETAS cert from about 2000.