if the locking /fuel filler flap is out of the way there is nothing to stop the vessel unscrewing. try leaving it on top of a radiator for a few hours having dosed it with some wd40 then unscrew. it is a normal thread ie unscrews anti clockwise.
Further to my last e-mail, I have a question, how does the base remove. The locking mechanism opens but it doesnt seem to want to unscrew, does it need a tap with a bit of wood and hammer or have I missed something.
I did look at the faq blog but that only seemed to deal with the locking mech and had me totally confused as to why a magnet is needed and what for :)