There's a quick mention of the filler cap drain on Capo's notes here;
http://www.ukmonster.co.uk/monster/s...ad.php?t=39677.
There were pictures on this thread at one time, which I have pdf copies of, but cannot post sadly.
Covering your cap with a big sticker will prevent it from breathing and eventually stop the engine.
It's most likely the drain as Rob and Jeff have pointed out.
If you can't blow through it or pass petrol down it, the best thing for unblocking is a guitar string. A wire wound bottom E string would be best. Just be careful as you push it past the rubber section at the top near the filler, in case you go through the side, but after that "roger away" as Capo once said.
Also detach the rubber hose that goes into a Y piece and leads to the ground behind the gearbox and check that's clear.
Then change your mechanics as I don't think they are that good if they can't change a block connector and say that old Monsters just fill up with water.