Mike
i have experienced similar prob a few times whereby you think you have brimmed the tank only to find you do cosiderably less than expected mileage. The 2 worst times were on the Lands End to John O Groats charity ride which really messed me up early in morniong with no petrol stations open in the highlands, and also down in the Sahara where there was a shortqge of stations. I think igts down to the tank design and getting an airlock in the part of the tank under the seat. Bloody annoying
Sorry for speelling this isnt a qwert keyboard here.
I have tried movong the bike, shaking it etc but one in ten it happens no matter what you do.