The engineer Doug arrived pretty much the exact time Vicky informed me earlier. He wore a facemask and observed all the current Covid regulations. He got to work straight away and employed a special power unit to power the underfloor heating and then a thermal imaging camera to locate the fault. It didn't take him long as he was very experienced and knowledgeable. It took some effort to break the tile to remove it as it was on a rubber crumb tile adhesive so absorbed the hammer impacts. Once he managed to get the tile up and expose the faulty wire he fixed it very quickly then demonstrated the heating working from the Thermostat controller. He tidied up any mess made and gave me some useful tips on replacing the tile. I replaced the tile later that day and left it overnight before grouting the following day then left it a further night before turning on the heating. It's now working well and is very comfortable in the kitchen again. It's not a cheap fix although cheaper and easier than trying to replace the underfloor heating by pulling up many tiles!