Larvae of the harlequin ladybird. The Harlequin ladybird is a non-native species originating from Asia. It has a negative impact on wildlife - out-competing native ladybirds for food and also eating their larvae and eggs. It is prevalent in towns and gardens. (Link)

Sounds like I should've killed them all.

Indeed, the larvae of Harmonia axyridis.

However, this species is running into some trouble here in the UK. Courtesy of the fact that sexually transmitted fungal infections, caused by fungi belonging to the Laboulbeniales, have jumped ship from the native UK ladybirds to the Harlequin ladybird. Of course, the native ladybirds have co-evolved with these fungi for a good few million years, but the Harlequin Ladybird hasn't encountered the UK fungal species until now. And the UK fungi appear in some cases to be doing a Covid-19 on the Harlequins.