Tr.9e, many thanks for the excellent group pics that include your Dad, and his welcoming letter from Sir Archibald Sinclair, the SoS for Air.
Incidentally, a Southern Railway Bulleid Light Pacific locomotive of the Battle of Britain Classis named Sir Archibald Sinclair is located at the Bluebell Railway and is presently being overhauled (end of Bluebell Railway plug!).
The posted image is too small for my poor eyes to read per se, but by right clicking on it and selecting Save As, I saved it to My Pictures, opened it, and selected the magnification icon. That made it readable for me. Less of a letter and more of a notice, but I guess that was as personal as could be expected at the time given the numbers involved.
Interesting that the importance of waiting patiently for one's turn to be called forward is down to filling vacancies in the schools which were being expanded greatly. It doesn't mention of course the wastage of ops and training that had to be made good before any further operational expansion was possible, but I'm sure that those waiting patiently in the slips had an inkling that was so.