| push it as hard as you can, with the change in European law from October 2003, manufacturers are on dodgy ground. The onus of proof is now on THEM, previously on the consumer, and as such they have to PROVE you have done something to contribute/make the fault. If it is a known fault, they are less likely to be able to prove it!
Some vehicle manufacturers are more twitchy to the Eu change than others, but your local trading standards should be able to advise and help more.
In short, providing you can prove any servicing work was carried out at the scheduled time and with OE parts, you will not automatically invalidate your warranty.
Paul | 
Hybrid Mode
