I am pretty sure that these wriggling beauties are the larvae of the Banded Hoverfly, Volucella zonaria. The adult female lays her eggs in the nests of social wasps where the larvae feed and develop on the detritus and wasp larvae within the nest. somewhere in loft-space, eaves or cavity wall you will have had a wasp nest.
Thank you; that makes sense in that we have seen a lot of random wasps in the house in the last few weeks and our eaves design is notorious for harbouring nests. It's feasible that there's one in the roofspace behind the bath. I've googled for more pictures or info about the larvae but can't find anything; do you have any clues, please?