Common cockchafer [Melolontha melolontha] has many nicknames - among them are maybug, dumbledore, snartlgog and Chwilen y bum in Welsh - the adults {up to 30mm in length} appear in April or May and live five to seven weeks. Females can lay up to 80 eggs each.
Historically, both adults and the grubs were major agricultural pests, but the arrival of modern pesticides has seen them become much rarer.
Actually, this was the first one I have ever seen - they make a very distinctive burring noise in flight and are attracted to lights from the tree tops. After taking some images I let it back out of the window.

en.wikipedia.org/wiki/Cockchafer