Caught at the scene of the crime
This is a Varied Carpet Beetle, Anthrenus verbasci, found inside a small picture frame where its larvae have destroyed three pinned specimens of Heath Fritillary butterfly. The butterflies were given to me years ago and I think came from Hungary - certainly not the UK, where it is an exceedingly rare butterfly and collecting has been illegal for many years. I've always felt awkward about these pinned butterflies, so it's probably a good thing they've now been dealt with.
This tiny beetle is the scourge of museum insect collections and taxidermy. In the wild it is found in bird and wasp nests - anywhere where there are organic debris to feed on. Our houses and museum collections provide easy pickings for them. If the numbers of larvae (shown in the extra photo) build up, they can do a lot of damage to carpets, woollen furnishing and clothing. The culprit and its offspring have now been destroyed!