you should be able to lock it without it being in the neck I think, some caps are really fiddly, I had one where the key would turn slightly in the lock once you took it off, then it took awhile to get it back to the right place.Thanks for the replies, I tried that last year and did no good. Living in Scotland I have had to do that several times over the past 30 years with other cars but with the duster it made no difference. If no one else has experienced this problem should I contact the dealer? Is there a fault? what moisture could freeze the inside of the fuel cap?
I even bought one of those plastic fuel caps from halfords which works fine but I find I can not take the key out of the original duster cap when it is not locked in the duster.