So I topped it up, and made a note to check on it an hour later.
At which point it had gone from 2.5bar down to just over 1.
Which means that either the boiler is venting water outside (bad) or one of the pipes under the floor has a really nasty leak (worse). The latter made even worse because we've had flooring put down, which means that any investigation of pipes is going to involve taking that up first, including removing whatever is on top of it. Joy!
So today is being spent doing lots of tidying up, as the flat is a bit of a disaster area, and then I'm texting our plumbers later on.
Wish me luck.
Original post on Dreamwidth - there are comments there.