Simon Glancey has submitted the following:

L.A.C. Close was killed when Armstrong-Whitworth Whitley N1405 of 51 Squadron was lost on a mission to Hornum. The Whitley was shot down by flak and crashed into the sea just off the island of Sylt, with the loss of all five crew members.
Royal Air Force Bomber Command Losses of the Second World War: 1939-1940; W.R. Chorley; 1992; Midland Counties Publications; ISBN 0 904597 85 7, page 32

Tony Hibberd has provided the following:

Whitley N1405 MU-? had departed Dishforth at 2035 for Ops Hornum. It was initially coned by 3/Res Flakscheinw Abt 369It was then engaged by lei Res Flak Battr 25/XI, hit and shot down. The aircraft crashed nr Hornum at 2345
This aircraft was the first Bomber Command loss against a German land-based target in WWII
Nacht Jagd Combat Archive, The Early Years Part 1 Dr Theo Boiten

Leslie Close is remembered in Burnhope on B153.01, B153.05 and B153.10

Airwar over Denmark
The CWGC entry for Leading Aircraftman Close