Window fitters caused damage – what next?

I will try to keep this brief. I posted on DIYUK – Link here to OG post last week about my sister changing the Windows in her sunroom (built with the house at the same time, option taken by previous owner who bought it brand new in the 1990s) from old wooden ones to new UPVC ones. When the window company was taking out the old windows in the sunroom, they took out all of it before fitting the UPVC ones in, in the meantime they said that the roof had come loose from the house – the crack shown in the video about 10mm – as it was not attached they then fitted the new windows but they've not complete any of the work apart from closing off the sunroom with the new glass panes and said it was too unsafe for them to go back in to complete the rest of the work without a structural engineer doing a survey.

We got a structural engineer survey done who bought along a builder and said that the roof of the sunroom was indeed not attached to the house, however the damage was done by the window company as they should not have taken out all the windows in one go and adequate support was not used. We are now waiting for the full report to come from the structural engineer in an email, but in the meantime the window company keeps ringing us because we have not paid them the full amount (of course because the work has not been completed) the structural engineer has agreed this was the right thing to do and not pay anymore right now.

I am at a loss as to what to do next because the structural survey person has said to me briefly over the phone that the Windows will need to come out and then the roof attached and then the Windows to be put back in. I envisage this to cost quite a lot of money, more so the time, I also don't envisage the window company to come out to take the Windows out to put back in what do I do in terms of paying the window company? I have about one third of the cost left for the window company to pay but of course with the cost of the structural engineer on the roof work what will my rights be?