If it was some botched DIY or something then I agree that you just have to lump it.
But aren’t boilers a bit different.
When we sold our house recently we were supposed to provide a load of paperwork to prove the boiler was safe and had been serviced etc. But the boiler had been fitted by an acuaintance of ours (who was corgi registered) who hadn’t given us the correct paper work so we couldn’t provide it to the solicitor. This mean the buyer had someone check it over at our cost.
I suspect if the vendors in this case have lied on their submission to the solicitors about this then they are in the wrong. The only way to know if you have any comeback would be to ask your solicitor.
However I suspect the answer may be “technically yes, you have a some comeback over the vendor” but to pursue those rights may be more hassle than having to pay £200 to fix it and move on.