Hi Fuggy
The recent block exemption rules applied to the Motor trade allow vehicle owners to get their cars serviced anywhere they like (as long as a vat registered garage!). The parts do not have be sourced from Mazda (for example) however they do need to be Original Equipment specification and quality. Most motor factors now sell parts that are supplied to them by the aftermarket arm of the same people who supply the dealers. So the parts are EXACTLY the same except for the part number on them (this is also the same in some cases). A dealer cannot legally void any warranty given by the manufacturer. If the cause of the particular fault is a part that is not standard or is not of the same quality as the original part then that is not covered by the warranty. (make sense?) .
If for instance you have the service done by an independant garage then you have a fault like the wiper motor not working the dealer cannot void the warranty on the wiper motor just because the spark plugs do not say Mazda on them. Some dealers do try it on and i await with baited breath the first one to be sued under these laws as it will teach the rest of the dealers that they are not anything special and in fact are pretty poor when it comes to quality of service. Remeber the dealers are franchised, most are not owned by the manufacutuer in question. If you have a problem with any particualr dealer regarding warrnaties then contact the Manufacturer customer care. If no luck with the first person you speak to then ask for the next person up and so on. These manufacturers rely on people taking the word of the first person they speak to. It is chjeaper for them not to pay anything so wil try it on.