Decoded the regular knocking noise from somewhere in the house to be a motorised valve actuator on the central heating trying, and failing, to move as it should.
Further investigation found that it rattled when it shouldn't. The other valve did this a few years ago, so I hada good idea what it was, and opening the case confirmed that a wee plastic nubbin had broken off, leaving the return spring untethered. A suitable diameter brass rod was cut to length and a groove filed in it for the spring to sit in. I mixed up a dab of Araldite and glued the rod into a handy hole in the casing. Araldite should be cured tomorrow, until then it's extra jumpers.