A friend told me I should squash it because as well as eating aphids and scale insects they also feed on butterfly and moth eggs and other ladybird species. I have left it alone. Last year I checked most of the ladybirds I found in our garden and the majority appeared to be Harlequins. If I go on a killing spree I am likely to have an aphid infestation and, to be honest, I don't believe eliminating them will make the slightest difference in the overall population. More will move in from neighbouring gardens to fill the gap in mine.
Many of the Harlequin colour forms look very similar to our own native species. Here are some identification guides that can help differentiate between them:
Harlequin Ladybird Survey - Recognition and Distinction has a useful PDF identification guide at
Ladybird descriptions_Info pack_NEW_v.5.pdf
The Ladybird Survey has an identification sheet showing the most common UK ladybird species and another for the larvae of UK ladybirds.
Ladybird Spotter has an interactive key to help identify ladybirds
A second updated edition (2013) of the Ladybirds Handbook is available through the publishers Pelagic Publishing and other booksellers.