After a certain amount of deliberation and further cross-referencing on the interweb, this is identified as Small Emerald, Hemistola chrysoprasaria. It occurs in the south of England & Wales around chalk woodlands and hedgerows where its foodplant, Clematis vitalba, grows.
Hold onto your pants and prepare for a flurry of moths!
|Field Guide to the Moths of Great Britain and Ireland - Paperback (2nd edition)|
|Paul Waring & Martin Townsend, illustrated by Richard Lewington|