Thanks @Seaitoverthere , and to others who have expressed sympathy.
Yes, Building Control have been round and were sympathetic and helpful. They have visited quite a few times so there was nothing alarming except for the extension roof. Builders told me it was water-tight, but this was clearly a lie as there are actual holes, and it's leaking throughout after the weather in the past few days.
I've found a project manager who will help me with the next steps, which is a relief.
I'm going to look into legal action to try to get some of my money back. But the loss has put me substantially over budget. Things I was told were paid for, actually weren't, so I will have to compromise on finishes, and maybe do the interior painting myself - that's pretty much all I'm capable of DIY-wise. My worry now is that the roof will need to be redone from scratch if they have bodged it.
The last week or so has been very difficult indeed, but in truth, the previous 8 months were spent consumed with worry over various issues, and, towards the end, the delays and excuses, and the impact on my mental and physical health has been great. In some ways I feel stronger now that the worst has happened.
I'm now thinking about temporary accommodation, as I suspect it will be several months before I can think of moving in. I had a mad idea to buy a flat rather than rent, as it would be cheaper for me (no mortgage), and no storage fees. But I'm probably not thinking straight right now.
In case it isn't clear by now - I am never doing this again!