The issue affects a range of JLR PHEV vehicles which seemingly all use the same heater. The full range is in this document; https://static.nhtsa.gov/odi/tsbs/2020/MC-10185391-0001.pdfJust been on the Evoque forum where owners with same P300e set up are experiencing exactly same problems.
The heater was replaced with new and car is now working fine.My p300e has had a day in the dealership today with their EV Technician. The "EV Temporarily Not Available" issue has been diagnosed as a fault with the High Voltage Coolant Heater (thanks FThewlis, your post certainly helped hasten the diagnosis). Again the part is on back order, a date couldn't be provided. As the works are expected to take 10 hours, mine is preliminary booked in for just before xmas when they have a loan car available.
Definitely Webasto - it’s certainly possible? Other manufacturers also make this type of heater.