Mother's CH is controlled by a Potterton EP2000.
For some time she has been complaining about the boiler locking out, etc. (and it does, and I know what to do to get that resolved). Today I had a revelation. The flaming unutterably stupid EP2000 has a battery. Any disconnection from mains, however brief, results in immediate loss of time and program settings. I think you can guess the connection...
Have now ordered battery.
Nothing whatsoever seems to indicate a battery failure to an ordinary user. It is a fairly odd Varta 3/V150H thing so cannot easily drop out to the shops to get one.
What a horrible controller. You need to shove a screwdriver up underneath to release the controller from the wallplate. Then, though you can see the battery, you have to prise it out with a screwdriver. Final insult, even at that point, there is no obvious marking as to which way round to re-fit the damn thing.