I was charged £470 for two visits and they didn't fix the the boiler. I contacted a different company and they said that they should never have touched an unvented cylinder without the correct qualifications. They fixed the problem and charged half the price. Luckily I had before/after photos and their email to prove how incompetent the company is. I reported them to trading standards.
If you pay by credit card then contact them. I got a full refund.