But when it whirred to a halt on a pot of flowers, it proved to be this instead. Hence it's Martin's Beetles today, and I think that this is a Cardinal Beetle, named after its resemblance to the robes of those gents from Wolsey to the Vatican conclave which recently elected the Pope. It looks like Pyrochroa serraticornis which has a red head, while its relative coccinea has a black one.
Here it is again showing its toothed right antenna which corresponds to the picture on the excellent website UK Safari. This does warn that 'Cardinal sightings' often turn out to be the even brighter red Lily Beetles, but I'm as confident as anyone of my dubious identifying reputation can be that this is not one of those.
The Lily Beetles have long been in the doghouse as pests of lilies, having arrived in the UK in the last century on imported flowers. UK Safari notes winningly that:
Their larvae have a delightful habit of disguising themselves as a pile of bird droppings. They do this by covering themselves in their own poo (don't try this at home kids).