So he owes you an EIC and if a scheme member Part P .......................... if not a scheme member he will need to see if your BCO will accept his EIC
I just record the readings I give out supplier each month and make sure that our bills reflect our readings. Not much point in over thinking this. My spread sheet covering 10 years shows that our consumption hasn’t risen - last winter it was less as it was a mild winter
Having PV and a wood...
Tripped circuit or RCD at the fuseboard?
Maybe the fan is wired to the lighting circuit?
Do you have any test kit and for the record a volt stick is not considered test kit