Given the current UK weather conditions I agree it is most likely to be the Webasto Diesel powered coolant heater. Location and description match the most likely source. Need a bit more info from the OP to determine what type of 'X' it is on, how long it lasts, and any other symptoms.

__________________
Me:
Current: 2011 E71 40D Vermilion Red/Beige Nappa

Wifey:
Current: 2012 R80 Countryman SDX Green/Cream