Thank you all for the kind and very helpful replies. The story about the loose battery connection was particularly insightful. I'll try not to tunnel vision too much on the engine and keep all options open.
Sorry for the late reply, been down in the dumps but my doctor doubled the dosage of anti depressant so i'm back up. Anyway, the day after my last post I contacted the garage about the temperature gauge and asked if he could have a look at it. He told me to bring it in the next day. A few hours after the call Boris declared lock down.
To cheer myself up I took the car for a drive, the next day. As I was driving the amber engine light came on. I drove past the garage and it seemed to be open so pulled in. I asked the owner if he could take a look and he said he'd sent the staff home and was going into lock down. He told me to bring it in next week. I didn't argue and got in my car to leave. As I was leaving I let him know that I hadn't paid for the window and he said he knew i'd forgotten and told me to pay over the phone. He also told me it was brand new, which was strange cuz the window was slightly scratched and muddy when I picked it up. I've taken some photos, see what you guys think of the job.
Anyway, I got home and did the usual checks: oil, coolant, orb reading. The oil was below the minimum so I filled that up with half a litre. The coolant wasn't halfway up the expansion tank so filled that up. The orb told me that there wasn't any error codes and that I would need some diagnostic tech specifically made for the car.
So as of right now I have a car with a mystery engine fault and I still need to pay the garage for rear window (will do that once my Universal Credits are paid in at the end of this month).