So first thing to do was expose as much of the petrol pipe as I could, the fuel sender, through the back bulkhead, along the centre tunnel, into the fuel filter in the engine bay and then to the fuel injectors.
The next stage was to turn the ignition on so the fuel pump would pressurise the system. Did this and after 10 minutes there was still no smell.
Next step was to run the engine, so did that and after 2 minutes still no sign of a smell.
Final step was a run around the block. After a couple of minutes the smell started and by the time I got back to the garage it was quite strong.
So checked around and there was no smell around the engine or the boot or the back of the tunnel, but a very strong smell coming up thought the front of the tunnel by the gearbox. But as far as I could feel there was no leak to create the smell.
So I now know the problem lies between the front of the tunnel, where the fuel pipe from the rear is connected to a pipe coming from the filter. Unfortunately that is very difficult to access so will have to consider what next.