We spent sometime riding around the Inshriach Forest near Kincraig yesterday. I'd been guided around some trails there a year ago and could remember some sections. We happily met up with a group from Inverness who managed to show us some trails and 1 1/2 that we'd found that they didn't. The less said about the 1/2 the better.
The 1:50 000 OS map is pretty poor, only showing the major forestry roads. Is there a map that shows the singletrack network? There is some great natural riding up there, a great fall back if there is bad weather and high winds making riding on the tops risky.