Seems that the trade is as confused as the public though, with most garages believing what the dealers say. They keep saying you HAVE to use genuine parts. However the likes of Vauxhall Trade Club parts are not O.E. they are cheaper aftermarket items, just coming from the dealer.
The Block Exemption Regulations (BER) introduced in October 2003, state that the car can be independently serviced, whilst still in the warranty period WITHOUT affecting the warranty. Providing that the servicing garage carries out the service in accordance with the vehicle manufacturers guidelines (including code reading where necessary) and providing they use MATCHING QUALITY PARTS. These can be the branded good quality parts from your factors. All the parts that meet the regulations are certificated. Again ask you local factor for the certificates from the parts manufacturers.
This gives you some profit opportunity from the parts, as well as the quality & peace of mind.