Thanks Inspectorman and Nigel. I hadn't previously read all of these VW self-studies, so it was helpful to go through them all.
I did charge the new battery overnight before installing to make sure it was full, and again on the car a few days later after spending time at home running it down a bit looking at the system. Our usual journey is the 20 mile drive to the town, It's mostly on a winding, hilly single track road and not above 4th gear and 30mph until we get to a main road for the last 2 miles. There are frequent stop/starts to let cars pass the other way, and to avoid sheep, deer etc. Maybe the charging schedule is similar to driving in a busy town? (The particle filter stays clear. So the engine must run suitably fast/hot for long enough.)
My multimeter is fairly upmarket, and DC voltage is specified as within 0.5%. But I suppose it could have drifted - I haven't calibrated it for several years. I used to work in a university electronics department , with good access to useful kit. (I once needed accurate voltage to challenge our home electricity supplier.) It does still agree well with another good meter that I keep on my boat. I know how instrument that display voltage aren't necessarily particularly accurate - one on my boat reads 0.2V lower than the meter across it and another 0.25V higher.
I may seem like looking for too much in this, but it's my nature to wonder if something a bit unexpected is a clue to a problem. The MOT is on 6th December, and I don't want tot risk being without a car. There is a bus twice a week from a bus stop 9 miles from home - not much use to us. So I'll wait until the sensor is replaced before doing anything else.