I am thinking that these are the sequence of event:
The UFH has failed.
The customer has contacted the builder.
The builder has contacted the UFH company.
The UFH company has given the builder the number of a person than can seek and repair the damage. If the damage is caused by a faulty cable then the UFH company will bear the cost of the repair.
The repair man has located the problem, removed the tile/tiles and made the repair. The repair man is independent and impartial.
He has identified that the problem is due to the end of the cable getting too hot and the glue has melted on the end cover. The end of the cable must be encased in tile adhesive or SLC. The adhesive and SLC draws away the heat from the cable. If there is a pocket of air around the termination block then the heat generated by the cable is not drawn away quick enough.
The repair man has tested the cable and found that the problem is caused by the installer - sorry.
The repair man gives the bill to the builder.
It seems to me that the builder thought that the problem must be down to a faulty cable but got a shock when he was given the bill (he got the bill because he contacted the UFH company).
In my opinion the builder should have contacted you as you had installed the cable. You would then have rung the UFH company and you would have been given a bill of £360.
The chances are that you would not have found the fault by yourself and you would have had to call in the experts anyway.
The builder has learned a lesson............a lesson that has cost him £360 !!!
You either split the difference with the builder or let this run and run.