Good luck with this. Having been someone who managed, by luck, to get good roofers etc. over the years, I got ripped off for around £10k on a building project at my house by builders who turned out to be nothing better than conmen. Apparently 25% of building work goes wrong - there are plenty of rotten apples in the building trade.
I am not legally qualified by any means, and my advice would go to CAB, but also go to a solicitor, at least for a consultation, if you think the money you lost is worth it (clearly solicitors are very expensive, but when I eventually went to see one, they were worth their weight).
Taking them to court is a tricky one. If they are complete conmen, you won't see any money, even if you have a CCJ - if they are legitimate business then you have a good chance of getting your money eventually as a CCJ against a business is generally bad for credit etc.
Typically if a workman doesn't perform the job to standard, then it will be expected that they will be given a chance to rectify, and this is often part of a written contract - if you have one. However, if you think they are conmen and have no faith in their work or their qualification to complete it to a satisfactory conclusion, you can consider that it is a "repudiatory breach" - something so fundamental that you cannot continue in the contract, and wish to terminate immediately, whilst allowing for suing for damages. You would probably need to be clear about what would constitute a breach like this, and this is where my solicitor was great. In my case, the firm had supplied false references, employed unqualified (and undocumented) labour, falsified VAT numbers on invoices, and clearly taken money without delivering the service. So I would dig deep around your builder and see what you can find - google, companies house online (if they are a ltd company), 192.com etc. are amazing tools at finding out information.
Clearly some of the work appears to be significantly below standard, but I would be careful at separating out work not done to standard, and work that is poor value for money.
I would advise to get proper legal counsel (from CAB or elsewhere) - and as I said, please don't take my post as gospel as I have no legal background, just stuff picked up during my issues. If you want me to give you pointers (phone numbers, web addresses) to places that may be able to give you information on the people you are employing, please feel free to PM me.