Mark,
Thanks for your advice. Yes, maybe
main tank - primer bulb - comp bottle - fuel pump - carb
is the way ahead. I would have to remove the venting I guess as the fuel pump would have to suck the fuel up to the bottle, rather than pumping into the bottle.. But any air leaks would render it all useless!
or what if the bottle was pressurised? ie..
main tank - primer - fuel pump - bottle - with no venting or return from the bottle - carb?
At the moment it is
main tank - primer bulb - fuel pump - comp bottle - carb (with gravity feed and vented return to the main tank).
or perhaps I should just wait until PB gets back from his hols!
best, Henry