This does not look like a red flour beetle, but more like some species of _Melolontha_, possibly a cockchafer. For more specific information, we would need to know, where your back garden is and maybe something to scale the bug. I would go for _Melolontha melolontha_ , the common cockchafer. Of the two other european species of _Melolontha_ , _M. hippocastani_ usually has a black edge of the elytra (see this picture), which this one doesn't seem to have, and _M. pectoralis_ is very rare and mostly (only?) found in Germany.