I'd be eternally grateful if ...

For anyone who maintains a vestigial interest in the minutiae of my travails, I just found it down the back of a radiator resting on the skirting board beneath, along with a nice lens-cloth, a letter from the DVLA, a couple of blank frames of 35mm film, a few dead woodlice and a great deal of dust

Sounds like you also need a housemaid

I guess Sirius's planned worked. Where you found it would certainly be the last place I'd look.

Was my 52 to 62mm filter converter ring there too? I have a vague memory of a tinkling sound behind a desk or bookcase sometime last summer thinking "uh oh that's going to be hard to get to".

Otherwise, maybe here. As likely a place as any other...