Thanks for your many comments and suggestions. I spoke with the helpful service folks. By getting me to charge with the 13A 3-pin type 2 adapter, then testing with their own Tesla wall chargers they suggested it probably was my wall charger.
our mains supply is reliable. NW electricity put in a 100A supply fuse for us last year. And they have replaced their transformer equipment and cables in the area in the past few years. We experience no service interruptions or any problems with other devices. My UPS logs historically showed long term VAC range of 230-250. When I’ve checked sockets with multimeters the results these days are usually around 240VAC +-5V.
I asked for more details on the interruptions if they were available from any vehicle logs. overvoltage? Undervoltage? Fluctuations? I was keen to get details so if I needed an electrician I’d have something more than ‘this car doesn’t charge’.
my concern was calling a busy electrician and having a non-specific, possibly intermittent problem for them. After all, the electricity to the 13A 3-pin socket comes off the same consumer unit, from an adjacent breaker to the Rolec’s.
they called back later to say that while investigating this they found something (unhelpfully, I forget what exactly) with the charging harness. Maybe their changes there might help?
After their tests, and mine, the battery was full. After a few local miles, I took a fair drive to a supercharger. First, to empty the battery a bit. Second, to test supercharging on the Model 3. Obviously my first mistake was to pull up at a non-CCS supercharger. I moved next to the only other car in ~8 bays, a Model S in one of a pair of CCS bays. Charged up for a few minutes (£0.92 - not seen the charging for charging in action before). Then back home. Got back with 149 miles of range. Plugged in the Rolec. Checking a few times early in charging I saw 32A and a range of 236-240VAC as the app values wandered. Seems OK. Overnight the battery charged to 90% with no obvious interruptions.
charging harness? One-off uninterrupted charge? I’ll check back with news when we’ve emptied the battery again and had another go at charging.