One of those 3 engineers if not all of them could have written you a report, and it should be those reports that form your evidence in court, the report should advise the tests which should be carried out, and the items that would be replaced as part of a service.
The 3 engineers that turned up also didn't need to replace the boiler, they could have fixed it by replacing the seals and flushing the heat exchanger, but instead chose to replace the boiler, so was there further issues with the boiler?
I had a similar issue with homeseve a couple of years back, each time an engineer arrived the boiler was made worse, 1 engineer came out twice spoke to my and my wife like crap and left the boiler not working so we refused to have him round any more, they still sent him so we wouldn't let him in. After 3 months in the height of winter with no hot water and electric heaters homeserve changed their terms and conditions and decided they will no longer support my boiler brand, and advised me there was nothing else they could do to fix my problems (upstairs rads not getting hot), I also tried a couple of heating engineers who were just as bad as the homeserve reps, I fixed the boiler and the fault myself. And swore never to use these firms again and to save the £30 per month I was paying them.
