Everyone has plumber horror stories to tell, and certainly you should be guided by something more than cost. A good plumber is an engineer, craftsman and psychologist all rolled into one. He or she is there to help you plan the most intimate room in your home. Don't just hire the one that looks the cheapest. Instead, it is fundamental that your plumber is reliable and always calls you back when you leave a message on the answering machine. This is important because almost certainly he or she will have to have a break in the job after the 'first fix' to allow the carpenter, tiler and painter in, leaving you to ring, maybe weeks later, to return for the 'second - and finishing- fix'.
You should also listen to other people's recommendations; ask to visit a previous client, and see the workmanship achieved.
Also, make a mental note of how well maintained his or her vehicle looks, whether the plumber works with an assistant, and whether the business is VAT registered. All these things will provide clues as to the calibre of the person you are dealing with.
There are things a plumber will want from you too: a plan; catalogues of fixtures you are intending to use; the names of the electrician and builder you are planning to use and what sort of timetable you have agreed with them. Very importantly, your plumber will also want a written list of all the individual tasks that will make up the job, not just the plumbing, but a list of all the jobs for all the trades. This is a written specification (the 'spec'), and something that all the artisans should be issued with. As with kitchens, bathrooms can grind to a halt if one set of people is waiting for another lot to do their job, so try to keep them informed with any updates to the specification and even get them all together to agree what happens when.
You are also dependent on the manufacturer delivering on time. Again, keep everyone informed if they fail to do so. And finally, a useful tip: listen carefully to the plumber's comments when you first meet and revise your specifications if need be. Agree on a price and only renegotiate if it is fair to do so.
This is where all your hard work in planning the potentials and pre-empting the problems will pay dividends. The layout that you have worked so hard to design, and the written specification are the blueprints by which everyone else works, and once the bones are in place you can start to think of the surface details.