Well, to be honest it was a real chore. We suspected the little seals and the corrugated plastic pipe from the get-go, so we replaced both, but that didn't cure it first time.
The seals are very delicate and one had been damaged on fitting, so I still had the leak! We ended up doing all sorts of stuff before we finally went back to look at the seals again. I eventually found it by taking the car for a bit of a drive round some bends and then stopping and shoving my hand up into the area where the filler pipe is inside the rear wing... that's when I felt the leak dripping.
The fix took all of 30 minutes!