We had a tough ride of about 20 miles. Through forest, across Salisbury Plain, but worst of all, down deeply rutted and muddy tracks. Getting through the mud was tough, 1st gear and sometimes slipping the clutch as I jumped between ruts. I did think , keep this up and the clutch will be shagged. Anyway much later coming off the plain, there was a huge hill up into the forest. Nearly got to the top and clipped a rock or stump, got knocked off line and stalled the bike. Bump started downhill, turned around and tried again. Nearly got to the top when I lost drive. So there was no slipping, no smell and no big noise prior to the failure. The bike did have 7k miles on it.
I think my technique through mud was the root cause. I need to attack it, plough through it, go over the top rather than drive slowly constantly trying to find the best line.