Have another update for this mystery!! I keep hitting dead ends here! I received this email from the Royal Air Force Museum.
Thank you for your enquiry, which we received on 3 April. These aircraft were part of a group of 9 that attacked a convoy of 7 merchant ships and 3 flak ships 15 miles south of IJmuiden. It was a daylight operation, so they would have taken off at 1150 am British Double Summer Time, and both were seen to crash in the sea after being hit by Flak. It is therefore highly unlikely that they would have flown over Groningen.
Yours sincerely
Peter Elliott
Senior Keeper
Department of Research & Information Services
Royal Air Force Museum
Hendon
London
So it looks like we are back to square one!!