Jack,
Start here maybe http://www.on-one.co.uk/ as this is the On One site.
The single speed in my ad is now listed on Ebay,my reason for advertising it on Ebay is that I think it is a pretty specialised spec,and maybe will only have limited resale potential. Afterall its a single speed , rigid forked, hardtail. Its like a 7! Pure and lightweight with no fancy extras! Ebay seems to have a number of the bikes on sale at any one time.
On one have made these bikes for a few years and they were one of the first manufacturers to produce the single speed range. Bike mags generally like the product range, many of their models feature as best buys in tests.
If you would like to know anything please drop me a blatmail.
Regards
Rich