The older engines (XUD) are more resilient to this mix than a HDi, simply as the pumps are different.
Having said that even an older IDI engine pump will not take just petrol as derv is needed for lubrication.
I used to do what Citronut said, we had a pump especially for just this, used to plug it into the fuel line on the bulkhead and drain it out.
In theory you should drain it all and refill with diesel (and change the fuel filter) but if you wanted to chance it with just brimming it with derv you can at your own risk obviously

, as a personal opinion, as long as there's less than about 10% petrol in the diesel it should be OK but don't hold me to it, stranger things have happened.
Andy.
91 205D-Turbo, gone but still missed
02 106D, TUD5B