Nick,
I have not found much more than this site:
http://www.goatpark.force9.co.uk/tempsite/gstrailer.htm [note: this site was taken down - see the cached version on the Internet Archive]
Plus see my
G503 album some shots of a 10-cwt Mortar Trailer I took at Beltring this year.
P.S.: luckily I got a 20-cwt trailer, seeing one already needs a 10-cwt trailer for two kids
