Underfloor heating fault at my mums

[ncoups]

My mum had underfloor heating stuck in her new bathroom (by a builder against my judgement) in summer but hasnt really used it much untill last week.
It worked for a couple of days and is on the blink now, the programmer/stat is all in order, but the floors not heating up.
I've checked continuity of the element and it seems fine, low reading.
Any ideas?
Cheers!