As changing the fuel pipe (See here) did not cure the petrol smell it was time to look elsewhere. I still think the leak is at the filler end so it is either at the filler cap or where the pipe meets the fuel tank. After my recent road trip I could see the streaks down the back that showed that she was definitely leaking at the cap, so even if that wasn't the source of the smell it needed investigating.
The actual seal looked OK, but the cap did seem to have some play in it so maybe the seal needs packing a bit.
So as a first experiment I've added a large rubber elastic band as a temporary test. I've no doubt the fuel will very quickly eat through it, but it will show if that is the source of the smell.
Postscript: Filled the tank today and went for a run. The elastic band worked wonders. There must still be a small leak, probably at the pipe/tank junction, as throwing the car round a left turn still generates a smell. But it disappears very quickly. And there are no streaks down the back of the car.
Job done !