The indemnity policies are used typically when Building Control approval should have been sought for a house project and it wasn't. They indemnify in the event that Building Control find out and force a rectification.
The elements of a heating project that require Building Control approval are the ones about heat-producing devices (boilers) and the thermal efficiency of any change. Also drainage (sewage) is subject to Building Control approval. Elements like the person who fitted the gas boiler being CORGI-registered (it isn't called that anymore - can't remember the new name but it's the same principle) are also a must.
Your buyer presumably doesn't (yet) know that you dont have a Building Control approval. Depending on what was changed, you may be able to have it inspected by making a BC application. But if lots of things are covered back up, that won't work.
I suggest you are going to have to come clean with your buyer and ask him what he will accept. He may yet decide he doesn't wish to proceed.
Plumbers can't self-certify thermal performance or drainage integrity - it has to be inspected.