Just a quick and hopefully final update on my rear brakes.
I got my first puncture last week in the rear (Kevlar tyres or not this nail still got in!) so I took the opportunity to have another look at the rear brake. Basically I tried a bit harder to mount the 160 mm bb7. But really it was never going to mount well, ie have the caliper grab the disk correctly without significant modification of something so i have gone back to the 185mm. Which is still working fine and locking up the rear wheel whenever I want.
Anyhow given that Yuba seem to have changed the rear disk mount point in recently sold frames my experiences with rear discs are probably a bit out of date. Check some of the other Yuba links in my blogroll for pics of the new offset mounting point, and some pics of some great new mundo creations!