EGR heater/coller failure
Hi, I had the same issue last winter, but I am from Czech Rep, winter here are colder then in UK, so my wife pointed on very poor heating and finally temperature jumped up to 130 and error message on screen shows and car heating was totally down as well.
Important is to check coolant level in reservoir. If level is deep down under minimum, or even missing coolant in reservoir, you are facing severe leakage of coolant. I had to add more then 1,5 liter of coolant in reservoir, start a car and all was normall till next episode...
Problem was in small heater/cooler unit on pipe from turbo exhaust to EGR valve. There was micro-crack between exhaust and coolant . And thru this crack can exhaust presurize your coolant out of system. Interesting thing - there was way how to simulate episode of this failure, just from time to time I faced massive loss of coolant - 2 litres or even more, but without engine failure.
To repair this is easy - bypass this heater/cooler, for coolant. In fact this is used only for quicker engine heat-up. If this is the root cause, then repair is much, much cheaper then head gasket failure or even broken cylinder head.