Cockchafer (Melolontha melolontha)
They are probably far more common than you realise as they are seldom seen. Occasionally one might fly in to a window with bit of thud or, if you have a window open, they might actually end up indoors as, like moths, they are certainly attracted to light.
They are not the most beautiful of creatures, in fact they are pretty ugly and I find it difficult to find anything to like about them!
I collect all those that end up in the moth trap (often 20 or so), put them in a box and take them a long way from the garden and deposit them. Their larvae are big, white 'slug' like things that do an enormous amount of damage to the roots of plants, especially trees, so we reckon the fewer we have in our garden the better.