I assume you mean 18 December 1939 (not 1940). There is a description of that raid here:
https://en.wikipedia.org/wiki/Battle...nd_Bight_(1939)
There is mention of the Whitley here:
https://www.tracesofwar.com/articles...ember-1939.htm
"In the morning of 18 December, before sunrise, a Whitley of 78 Squadron took off from RAF Dishforth in Yorkshire. The aircraft crossed the North Sea in the direction of Helgoland. Arriving at 08:00 the sky was half cast over, which would provide the bombers the required coverage. A message was sent to headquarters of Group 3. The message was forwarded to the squadrons. Take off was to take place at 09:30. Over King’s Lynn in Norfolk the 24 Wellingtons were to assemble in formation."
However, I checked the F540 report for 78 Squadron for that date (using the preview function as Steve did) and there is no mention of any activity for the Squadron on that date. There don't appear to be any F541 reports on file for 1939 for the squadron.
UPDATE: I believe it was 51 squadron that did the weather recon patrol. On this link:
https://discovery.nationalarchives.g...ils/r/C2503133
go to page 32 (of 659). A weather mission (2 planes?) was planned to go from Borkum to Wangerooge and from Stylt to Wangerooge.
On this link, go to page 10 at the bottom.
https://discovery.nationalarchives.g...ils/r/D8451294
The mission was flown to arrive over their objectives at dawn.
Page 2 of this has plane ids and crew lists:
https://discovery.nationalarchives.g...ils/r/D8451295
Whitley IV K.9047
S/Ldr W.H.N. Turner, P/O A.C. Peach, Sgt R. G. Bruce, LAC D.S. Edmundson, A.C.2. Boyd.
Whitley IV K.9048
F/O T. K. Milne, P/O B.W. Hayward, Sgt J. B. Ritchie, LAC. A.W.D. Lyne, A.C.2. F Hargreaves