Went for a 200 odd mile ride with the local restorers club, they were all pussys and turned up with their new BMW's etc etc, not me as the BSA is my only bike (for now), BSA it was.
At basically the furthest most point from home I was cruising along at 65 M/HR and the old girl starting farting and surging, I got that horrid sinking feeling.
Pulled over and as I pulled up could smell that scary fuel on hot magneto smell, turned the fuel off and took a few steps back from the bike.
Long story in short, the main jet had vibrated loose over the last 800 MLS and stopped the fuel going through the carb, tightened up the main jet and she started first kick.
Morale of the story:
* I had noticed the idle characterisitics of the bike had changed over the last couple of rides, indicating there was a problem, yet I ignored it, if something changes, find the cause.
* Loctite absolutely everything on these bikes.