I am reasonably sure that your father witnessed the crash between two 218 Sqn Blenheims on 18th August 1940
"On Sunday 18th tragedy once again hit the squadron,
when two aircraft of ‘B' Flight collided while carrying out
a formation exercise. Blenheim L.9264 flown by Pilot Officer William Wheelwright struck Blenheim T.1929 flown by Flight lieutenant George Newton, P/O Wheelwrights aircraft immediately went into a inverted spin
and smashed into the ground, F/Lt Newton's aircraft caught fire in the air and dived into the ground and exploded, there was survivors."
See:
http://host.raf38group.org/218squadron/reform
I am unsure where the aircraft actually crashed but William Wheelwright death was registered at Cambridge so it had to be in the vicinity. 218 Sqn were based at Oakington in Cambridgeshire at the time
Crew L9264
Wheelwright, W B : Pilot Officer : Blenheim IV L9264 of 218 Squadron, 18-Aug-1940. Pilot, Killed
Bilton, J N : Sergeant : Blenheim IV L9264 of 218 Squadron, 18-Aug-1940. Crew, Killed
Clapperton, R W : Sergeant : Blenheim IV L9264 of 218 Squadron, 18-Aug-1940. Crew, Killed
Crew T1929
Newton, George Edward : Flight Lieutenant (37824): Blenheim IV T1929 of 218 Squadron, 18-Aug-1940. Pilot, Killed
Smith, W L : Sergeant : Blenheim IV T1929 of 218 Squadron, 18-Aug-1940. Crew
Dennis, E A : Pilot Officer RAAF : Lancaster III ED753 of 50 Squadron, 25-Jul-1943 / 26-Jul-1943. Pilot, Killed
Harrison, R H : Aircraftman 1st Class : Blenheim IV T1929 of 218 Squadron, 18-Aug-1940. Crew, Killed