The problem with your door is the central locking motor, the spring that pulls the handle back down has probably broken. Its easy enough to change (only 1 spanner in the haynes manual).

You can get a new petrol cap on Ebay for under £10 posted..

http://cgi.ebay.co.uk/Locking-Fuel-P...item3a4caea56e

Or go to Ford and they can supply a genuine one with the correct barrel to match your current key.