After a fair bit of driving I figured out that the petrol smell only occurred if I went round left turns very fast. I had been doing that a fair bit as it was such fun. I assume that when I do that all the petrol in the tank slops to the right side of the tank, where the filler system is located. The previous owner confirmed that the petrol pipe between the tank and the filler cap was a bit of a bodge and might be leaking. In fact it turns out it was made from a couple of rubber gaiters off suspension units.
So the first thing I did was replace the original hose with a proper brand new one.
As it turns out that did not help and the smell still occurs in fast left turns. But I was pleasantly surprised that it fixed the ‘slow-filler’ problem I had before. Now fills up as quick as every other car.
Status: Still need to track down the leak