Thanks for all the help with this. It's hard to find decent Rims for this in UK apart from the Sun Ringle one on SJS Cycles site which is a little on the pricey side.
The cheap single-wall rim at SJS is perfectly decent. A 36h 20" wheel is completely overbuilt for normal use. Even a basic rim makes a bomb-proof wheel if the builder is even moderately competent.
As another source of rims, have you tried Pashley? I'm out of touch, but Dan Farrell used to be the man to talk to there for Moulton spares. They're very helpful, in my experience.
Thanks Jim, the reason i am looking for something better than the SJS Cheapish single wall Rim, is that the cycle is going to be used off road by the owners, track with pot holes etc as they live in the middle of nowhere; they've asked for MTB Tyres, Slime tubes and we're even putting Kevlar tape in too on the finished wheels (OVERKILL?)!!