Hi Ian, i had a similar problem with the mono tank on the rolling chassis i sold a couple of months ago, lost key so couldn't open it.
I removed all the allen bolts (6?) , then i could lift the whole filler assy a bit at the front, enough to get a screwdriver in (covered in a bit of leather to protect the paintwork etc), and managed to pry it up and out without damaging the latch assy (you need to be a bit 'firm' with it). then took the whole thing to my local locksmith who , within the hour had provided me with 2 keys for the princely sum of £15.