This is a Birch Shieldbug (
Elasmostethus interstinctus). The adult is like a smaller version of the Hawthorn Shield Bug. It is found throughout the British Isles in mixed woodlands with Birch and it is generally common where it occurs. It hibernates as an adult and eggs are laid in May and June. The new generation becomes adult from August onwards. The egg is pointed and laid in batches of up to 24, usually on birches with catkins. Aspen and Hazel may also be used.
Because it hibernates it often comes into the house at this time of year. I had one arrive in Brackenside one October and this one came into The Willows today.
Perhaps rather cruel but I put this one on its back for a minute to show you the underside. It soon righted itself without my aid.