Just a bit of follow up on this....I have not had the chance to replace the fuel filter and the grass needed to be cut, so I wrapped the fuel line with aluminium foil and hoped for the best. It actually just about solved the problem! No hissing or tank expansion when I was done and peering into the tank I barely noticed a bubble or two coming up from the bottom of the tank. This was after 1hour and 45 munutes of running PTO and full engine throttle. Filter change and possible re-routing of the fuel line will be on my to do list this week though!