a pal of mine wanted a bathroom fitted and various tradesmem and looked around but none had actually turned in a quote so he went with Dolphin. Be-suited salesman turned up, went through the motions of measuring and designing and then, on the night quoted £60,000. When faced with guffaws, he offered him 50% off if he signed there and then - at which point he was ahown the door. To be fair it was more than just fitting a bathroom so was never going to be cheap, but even £30k was stupid, and to quote £60k then offering 50% off when you refuse is just an insult to your intellenge. Eventually he found a sensible local builder who did a perfeclty fine job, not cheap but fair enough price.
The moral is you need a normal local tradesman you feel you can trust - personal recommendation si the only way to go - and once you get (say) a plumber, he'll know a tiler, who'll know a decorator and you're in. Be clear on scope of work, itemised, so then the inevitable extras are fair enough and clear to you both. Don't mess em about, but be clear and fair, and pay up promptly. Next job is then easier and more laid back.
I got a guy in to fit a fireplace - strict scope and price. He subsequenlty did my bathroom on a "ballpark" estimate with lots of scope creep. When he fitted my new kitchen I didn't even have an estimate - just let him crack on - as we had built up trust by thjs point. Bill was fine - more than my hopes but a lot less than my fears given how much work it was.