'98 rover 216 petrol cap

Hi, just bought a (very) second hand 216 cabrio, but the petrol cap doesn't
pop open when i pull the lever in the cab of the car.  Is there a particular
way of opening it as it is more luck than judgement at the moment.  we have
got it open using various combinations of opening and closing the central
locking while in the car, keys in, keys out, legs crossed fingers in ears
that sort of thing...
It hasn't come with a manual of any kind, so if there is anyone out there
who has the same car, can you please help me out.

Thanks in advance for what i am sure is an easy answer to the problem.

Nick