I was passed the folloing info from the original letter writer. Someone had visited Kew...

AIR29 piece 413, titled 'Photographic Development Unit Form 541'

"AIR 29/ 413 (which continues, in spite of the change of unit name) notes that X4645, fitted with Merlin III No.31599, was delivered to the PRU at Heston on 14 Nov 40.

Two days later, on 16 Nov 1940, the same AIR29/ 413 notes yet another change of unit name - it became 'No.1 Photographic Reconnaissance Unit' from this day forward. [No.3 PRU was formed the same day, but it was at Oakington in 3 Group Bomber Command].

Strangely there is no mention in AIR29/ 413 of an accident to X4645 on 17 Nov 40, but it is quoted as 'Cat 2 17.11.40' - which of course implies an accident - in Air-Britain's Aeromilitaria, Spring 2009 issue, p.31, in an article titled 'Spitfire I - Final production, Part 2'.

On 23 Nov 40 the AIR29/ 413 lists X4645 and Merlin III 31599 as being sent to the CRU (I believe this translates as Civilian Repair Unit) at Cowley."