Hi Dave, I am also new to the forum, even though I have had my fiesta since 1998, but have had the same problem so have jumped in to give my tuppence worth.
If the tank itself is not leaking (not sure from your post if it gets worse with a lead foot but leaks all the time, or only leaks when the engine is on). There is a small length of flexible hose from the pump which leads to a length of steel pipe leading to the fuel filter (round canister at the front lower left of the tank) in my case this was very rotten it may be the same on yours. Not sure if you can get to it without taking the tank off, as my tank was off anyway to weld up the inner wing and sills. It's only 4 bolts to drop the tank and mine came out without too much trouble. (one captive nut was scrap so was replaced with a standard nut and bolt.) If it is the tank it is probably around the filler neck area.
You can get a second hand tank for around 10 to 20 quid but swapping over the pump and feeds is a pain so may be better (if you need a tank) to get the whole shooting match from a scrappy or ebay. I also replaced the length of steel fuel line with plastic (get fuel grade plastic line if you do this or it may melt)
Hope that helps, good luck

Ian