This moth is a great oak beauty (Hypomecis roboraria) and Nationally Scarce (Nb) species with its main strongholds in England in wood areas in the south east.
This was on our lighted kitchen window pane and, in this underside view, the diagnostic pale spots with dark surrounds at the wingtips are clearly visible.
The moth is widespread across Sussex, though less so in the east, and the larvae, as the English name suggests, feed on oak.