I don't have a marine tank (yet, maybe some day

) but a lfs to me has a big marine section. Its maidenhead aquatics in bristol, i don't know how near they are to you but they are a chain in wyevale garden centres. I think they are all run separatly so they might not all do marine stuff but the guys in the bristol one are really helpful so i'm sure they could point you in the right direction if they don't have it themselves in your local store.
Have fun, marine tanks always look amazing, i love the way the anaenomies (sp!?) sway, really peaceful,
Rachel