Sorry to answer my own question but there is a 400 x 18 Avon Road Rider 2 available, to save the metric uncertainty. £87 inc carriage at BikespeedUK.com. Seems reasonable, and I`ve used the Roadrider before on other bikes, with good results
I looked at a lot of data and it looks like 350 might have been ok, but I will go with the 400
Thanks, Rick