If the car would not start then the £20 of petrol definitely did have a detrimental effect, so you want your full purchase price refunded. Do not accept the car back. You can sometimes get away with up to 9 litres of petrol added to a 60 litre diesel tank because the petrol 'floats' on top of the diesel and all you have to do is prevent it getting picked up by the pump which takes the fuel from the lowest point in the tank. But if the salesman put £20 of petrol into a near empty tank instead of diesel and that got picked up by the fuel feed then the petrol will have destroyed the injection pump and injectors and there is potentially £4,000 - £5,000 worth of damage.