Cowboy builders

If you ask Age Concern they should have a list of approved tradesmen who will not 'rip off' older customers and will do a decent job.

Surprised at Facebook Maybe broaden your search.
Ask around friends neighbours or if you see a new job done perhaps the people would be kind enough to give the builders name.

Here is a reference to a Guardian article on finding a builder, which lists a number of the online sites and tells you something about them
www.theguardian.com/money/2018/jun/04/reliable-builder-websites-traders

Unfortunately in their test run they used a query about garden services, just about the most difficult people to find. I did find though that the day rates they were quoted per person, were similar to what is charged in Oxfordshire. So tradesmen on these sites quote competitive rates.

I have heard that there is also an online site where you can put in what you want done, and ask tradesmen to quote.
Not sure if it is nationwide though.

I can’t remember what it’s called but there is an online trade directory with reviews and awards etc I ve used it twice and both times was very happy with the outcome I used a highly recommend chap for my bathroom from a workmate and wasn’t so happy

Ask around all your friends and neighbours. You will hear the unvarnished truth.