This ladybird is unique in that it shows a large variation in color patterns between individuals. The following photos, all taken in Hull, illustrate this.
Harlequin Ladybird Survey, and I reported these sightings. Data sent by members of the public (in the form of records and photographs) have helped to follow this invasion in great detail. This series of maps from the survey's website show the spread of the Harlequin in the UK year on year.
There is lots of info from the website, not only on the Harlequin, but also on the native ladybirds. You should be able to identify not only the adult ladybird, but also the larva. Here you can compare it with a 7-spot larvae.
Harlequin ladybird larva (with two lines of reddish spiky tufts on top of abdomen).
A 7-spot ladybird larvae (only black tufts on top of abdomen). Tufts are not obviously spiky.
Should we be concerned? Despite the Harlequin being a quite handsome bug, there are fears its explosive spread could result in threats for other ladybird species, either directly or indirectly. They are reportedly very aggressive toward other ladybirds and even humans! Although I must say I have often collected them by hand and they seem not to be aggressive when being handled carefully, in the same way the 7-spot. They eat larvae and eggs of ladybirds if aphids become scarce. Their populations might not be kept in check by predators and parasites. Also, they hibernate in houses in a communal way and they can become a nuisance.
In Hull, the Harlequin became very common in 2007, but didn't see many in summer 2008, maybe the gloomy, wet summer didn't suit them!