Nice to come across someone from the next valley on here! As you're from Hudds you'll probably know the main dealer there is Perrys. I've had mixed experiences from them, which is pretty much the same as any of the local main dealers. I'm not aware of any nearby Alfa Specialists apart from Betacar in Cullingworth, who are really Lancia specialists trying to branch out. Theres also someone in Keighley who's mentioned in the AROC directory of Services but I haven't tried them.
Consequently I take both Alfas to a capable, honest and trustworthy mechanic who runs a well respected local garage. He listens, fixes the problem and charges a reasonable amount of money, and I'm very happy! He's just done a service on the 156 and the car feels great. Occasionally I'll bite the bullet and take the car to the main dealer but it never returns feeling like they've done anything. I usually have to be desperate before I'll take it there.
Best of luck sourcing your new 156.