Sunday morning before-breakfast ride, bike comes to a halt after about 7 km. It's just onto reserve but there is surely gas in there. Wouldn't start until I opened the cap, then all was well, filled up, rode home.
Found a good write-up on vent valve removal:
http://forum.cog-online.org/index.php?topic=60228.msg466823#msg466823
and deleted the lower little valve.