As David says, the PSV enthusiasts saved much material via the Kithead Trust who have listed whereabouts of surviving records.
http://www.kitheadtrust.org.uk/
There doesn't seem to be any later 1930s Middlesex material. The Armed forces block allocations were of course never used again after 1939 (It wouldn't work these days, some poor clerk in the MOD would have to fill in SORN (Statutory Off-road Notice) every year.
Nigel Watson's Carrier book has quite comprehensive Reg. No. / Contract listings. Does anybody know where he got them from ? Bovington springs to mind if they are not at Kew.