This could be anything but certainly is a problem. Warmup PLC have a tehnical team tat crawl around the country fixing Warmup underfloor heating systems so my advice would be to tell Mrs Customer to give Warmup a call and get their tech-guys out to check the system and obviously not to use it before they check it.
It could be a damage in the cable... if it is you can fix it with little effort. It could be a power-related problem or it could just be a faulty stat.
My advice to others in the future. Use
Electric Underfloor Heating Systems - Varme UK (buy from
Electric Underfloor Heating UK DIY | Homepage for Handyheat or
Ambient Underfloor Heating ) or
Flexel Heating Solutions - Under Floor and Ceiling Heating Solutions (buy from
Online Electric Underfloor Heating Specialists or
underfloor heating UK). And on the thermstat front Heastmiser or Aube seem to be the two best in the maret at the current time.