This is spot on...
For my sins I’m a senior structural technician (old school draughtsman) with about 30 years behind me. My immediate advice is to get a local structural engineer out to asses exactly what is there and what’s not there and put a plan in action. Building Control will ask you to get an engineer on board anyway so that would be my first port of call. I would then instruct the builder to implement the structural requirements as specified by the Structural Engineer written instructions.
You may need to move out (I would), as others have previously stated. Nothing worse than a builder who’s been doing things wrong for years thinking they know all the answers it really is frightening.
The good news is that it’s still standing, this can be just pure luck at times and it’s just a matter of time before it fails.
A set of calcs should be around £250 and retro building notice from building control is £120.. here in N.Ireland.
You can also build a false chimney, the Glass reinforced plastic type if you are worried about appearances and aesthetics which will drastically reduce loading in your attic, but it still requires support.
Good luck.