For the first three years I would use an Alfa Dealer. I don't care what the 'block exemption' rule says, if anything goes wrong under warranty Alfa Romeo will be more keen to look after you than if you use a specialist, let alone any garage 'round the corner'. £400 sounds expensive though for a 24,000 mile service. My 48,000 service cost me £300, and that included extra work including cleaning and lubricating the gear linkage. I'm not sure if Alfa dealers charge all the same - shop around, mention you're a member of 'this and that' club, ask for a discount, you may be surprised!
