We got our first diesel brand new last november. Not had it long and the wife put prob less than half a gallon of petrol in, then brimmed it on the advice of person at the petrol station she reckoned. It has now done 14k miles with no probs so hoping we've got away with it but you got me worried now!
Edit: from AA website:
Generally a small amount of incorrect fuel should not damage the engine as long as you have not started the car and top up fully with diesel fuel.
If you've added more than 10% (5 litres in a 50 litre tank) petrol – Drain the tank and refill with diesel
If you've added less than 10% petrol (5 litres in a 50 litre tank) – Top up with diesel and run normally unless the manufacturer has advised otherwise