According to my book of moths this is a Mottled Beauty which is common throughout Britain and varies in colour.  It was probably disturbed while we were pruning some bushes and has been on this wall most of the day but no doubt will fly around when it gets dark.   The moths emerge in June and lay eggs in July with the caterpillars emerging shortly after. They hibernate in winter then pupate in the soil in May before completing the cycle in June.  Although the Red Admiral butterfly in the garden was much more colourful it was poorly focused so the moth was second choice.