I,ve done this in a few houses, you will need a liner fitting and really the fitter should be HETAS approved, this means that you get a tag fitted near to the utilities, and it passes all relevant building regs.
There's quite a few idiots and blaggers out there, I had one local guy measure my latest install using his thumb with one eye closed from a distance of 80 feet. (he was about 12 foot out in his favour). We didnt use him.
If you want heat, go scandinavian, the latest UK carron range are okay but to be honest arent a patch on products like skantherm.
I used a local fitter and they were okay but really messy, if you need any info including who to avoid then please do dm me.