Sake.. roofers/builders are absolute ***** on this sort of thing.. :rolleyes:
Gas engineer done the right thing - Not only unfortunately for you, but for him too, as pointed out above.
Boiler - It's balanced flue, so no 'plume management kit' can be added, and anyway even then a new boiler in...
Yes, builders become responsible here. They'll try to argue that your flue shouldn't plume over neighbours property, but still their fault.
Put up a pic, and say what boiler make/model is please.