Sorry, dumb one really. I just had the heating out for a couple of days and eventually found a fuse blown in a controller for the (LK) underfloor heating.
I can't tell what rating it has (amps). It has T200H250V embossed around the end, and is made by ESKA with an S on the bottom/top. But that doesn't Google well.
I can see that the transformer it was (presumably) protecting has an output of 2 x 20VA @ 12V, I've found a 3.15A 20mm clear-glass fuse for sale, but maybe someone on here can better guess than me?
End of question.