Hi. Thanks for the replies.
mtskull is correct I'm afraid. With a modern common rail diesel petrol can cause a lot of damage.
I did drive my transit back home, which was only 5 minutes away, and subsequently read that that was the last thing I should have done.
I emptied the tank as much as I could by disconnecting the filler hose, filling a bucket with the fuel and decanting it into drums. I siphoned out the very last bit that was below the filler connector on the tank.
This was on the street, as I don't have a garage, and I'm sure that 2 out of every 3 passers by would stop to tell me of the time that they did the same thing to their diesel.
Anyway, I filled it back up with diesel and it started 1st time and seems to drive fine. When the tank was half full I brimmed it again and have had no issues since.
Apart from 65 litres of mixed fuel that I need to get rid of.