@thickbed can I assume that if we are still going on about this now, that they did indeed contact you before they went ahead with the repair and you refused to go look at the job or said it wasn't your job to or something and now you're just trying to pass the blame?
Reason I say this is because if you didn't have the chance to repair a fault of yours, or at least be involved in agreeing whether repair costs are appropriate, it's not something you need to carry on worrying about.
And you keep skipping past me pointing this fact out.
So I'm starting to think you perhaps did have the opportunity but avoided it maybe?
If that is the case, just split the bill with the builder and swallow your pride and move on.
But don't wriggle out of it.
Perhaps go on a UFH installation course and learn about it. Or avoid installing it in the future.