Roo,
We had to get our central heating system replaced including the pipework so the only extra cost you'd have would be in getting the gas brought up to your house from the main street pipework.
For 8 rads with thermostatic valves, a new wall boiler to replace the baxi fire boiler, flushing, new pipework, a new hot water cylinder and a fire our cheapest quote was 6400. We went with this and I was very impressed with the neatness of the lads work.Originally we wanted to use the existing pipework and had agreed 5900 with the company. When they came to do the job, some of the pipework crumbled so they said they would completely repalce the piping for 500 quid bringing it to 6400. I think it was worth it though as we didnt know how bad the existing pipework was and could have been looking at having to get the pipework done again a few years down the line.
I think the 8 rads part of the quote was about 2400 to replace..