Don't know why but I got the owners manual out tonight and had a read.
For petrol engines Seat actually advise you to use additives. The book says,
"Petrol additives improve the quality of the petrol"
It goes on to say,
"The quality of the petrol influences running behaviour, performance and
service life of the engine. For this reason, you should use good quality petrol containing additives. These additives will
help to prevent corrosion, keep the fuel system clean and prevent deposits from building up in the engine.
If good quality petrol with additives is not available or engine problems occour, the required additives must be added during refuelling"
Under Diesel there is no mention of additives, but being pedantic one could argue that as they didn't say you shouldn't add additives then you can, especially as they recomend them for other engines.
My local filling station has a notice on it's diesel pumps advising that their diesel contains 7% bio-diesel, which is the maximum recomended for use in the
Seat Ibiza. I have noticed that if I fill up at a different
garage that is not blending it's fuel, I get between 5 and 10 mpg more. That suggests to me that the
Ibiza doesn't like the bio-diesel content and is why I initially considered using the Millers additive, to protect my engine.
It sort of opens the whole debate up again, do I or don't I. But as far as I have read and please correct me if I have missed something, there is nothing in the handbook that says "don't use additives"
I've used millers pretty much from from new. That's about 14 months ago. 
