Sorry to dispute, but NU only changed to Aviva in 2009, but lets not nit pic... it was definately their policy and they definately paid out. Presumably they wouldnt have done so if they didnt have to?
The Merc garage I took the car to, were insistent it had to be done that way, so I went with it (as I didnt have to pay). A friend reported that they had similar experience (petrol into a diesel engine) and as they werent covered, they took it to a local garage who just did the traditional cheapy clean out - they had recurring problems ongoing, though I cant be sure it is related.
I guess the beauty of comparison sites is that they give you an idea of what you should look to pay, but money saving isnt just about the premium but perhaps about the time and hassle you have to give the problem if it isnt covered, and I reckon that I will continue to use a broker, and pay a bit more for their expertise.
Thanks so much for your input.