Checkatrade is a marketing site for builders not householders I did check out the local listings only 3 listed and never heard of any. It’s certainly not any kind of guarantee of quality, we have a local admag the builders use there is good selection there.
It is normal for a builder to ask for part payment up front, some will ask for materials paid for by client. Many price the work according to the cost of the car on the driveway, everyone has been ripped off at some time. So unless you have a friend in the trade or a relative that’s good at DIY its pot luck.