Thanks for the replies. Would you recommend I get a report from a gas fitter/plumber or from some type of surveyor? Is there a specific type of witness recommended for this?
As for the what was expected, that is a good question. They advertised the fitting as free, which was untrue, and they advertised a "Conversion Pack" to take it from a system boiler to a combi, and all I can see has been done is some pipes joined roughly in the loft. I would suggest that having charged £700 for a conversion pack, it is reasonable to expect at least some works to change the pipework. I would say that given the fact that they gave me less than 10 hours total labour and charged £2,000 or so more than the parts cost, that there is something not quite right.
I have previously had them out as 2 radiators didn't heat up and I have also sent them a letter in April to complain and have had no reply. I haven't pushed it as I have been ill, so please don't think I have left it a year and then complained.