It appears it wasn't locked after all. A sharp tap with a wooden mallet was
all that was required
the lock is a sprung pin which locates in the fuel filler flap which then
swivels out on a rivet.
rare earth magnets from a computer should do it. the pin tends to get stuck
with coal dust so plenty of wd40 helps.
the threads do get tight once the flap is loose leave it to warm up and try
tapping with a wooden mallet.