When I gave the car to my mechanic for the MOT I asked him to check out the petrol smell, as the car had been sitting for over a month when he came to pick it up. I had decided that the problem was that the pipe was absorbing petrol, which then evaporated off when the engine was warmed up.
Turns out I was totally wrong. What was actually happening was that as the pipes dried out the joints were opening slightly, causing an actual leak. When the pipes got wet again the joints swelled up and sealed the gap.
The big problem was that there was no insert in the plastic pipe so that when the Jubilee clip was tightened it compressed the plastic pipe and after time the joint would leak.
Haven't had a chance to test it yet, but he is confident that the replacement pipe will cure it.
Yet another example of a poor design decision on the car,