Thank you everyone for the suggestions. I think John's thoughts on silicon and Portholme saying about possible fuel contaminants could be spot on! Interestingly, Cobra agree with Ribtec1 about cutting a hole in the deck/tank and that's probably the route we'll go. Thanks to Whiskylee for the vent pipe suggestion, but we have run with the fuel cap off and the problem still persists, so I'm guessing it's not that.
I'll report back on what we find when the tank is drained/cleaned.
Thanks once again to everyone who posted.
You get what you settle for!