Yes, these trailers were 'assembled' by Pearsons and then the company acquired stocks post-war which they then sold off onto the civvy market. I was sent some years ago a copy of various classifieds by a farmer guy that bought a CMP or two from Pride & Clarke aka 'Slide & Shark', south London...P&C sold off Chevies and Fords as seen, but to go with your say former FAT for agricultural work, you could buy a host of ex-military trailers from numerous fly-by-night people.
This photo is I seem to recall a fomer military trailer.
For further pix see:
http://www.pearsons.moonfruit.com/
and to order a book:
http://home.btconnect.com/transpennine/pm03.html