Western Conifer Seed Bug
I'm pretty sure that this is a Western Conifer Seed Bug (Leptoglossus occidentalis). It has the characteristic white zigzag line across the middle of its wings and expanded rear tibiae. This large bug was sitting outside on the frame of my conservatory in Ipswich, Suffolk (TM166450) on 12 March 2017. These bugs were first detected in Britain in 2007. They feed on conifer trees.
PS ID accepted as correct by Tristan Bantock on iRecord, 13 Mar 2017.