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When did the brother have his fall? Because it sounds like the windows should have been ordered a long time ago to be done on time for the build?
While I agree that individual trades people can be excellent even if flakey, a loft conversion needs a lot of coordination and so not arranging things on time seems like a red flag to
me. I’ve had quite a few builders with
excellent reviews (even from neighbours) that were a complete disaster. I’ve also learned that if the communication doesn’t work at the beginning, it’s not going to improve!
Maybe you can have an honest conversation where you lay out how this puts you in a difficult position, and while you have a lot of understanding about his brother this has become untenable. When you paid the deposit did you agree a completion date? I would assume that if now completion seems significantly later due to the windows not being ordered you could try to get the deposit back, but defer to the experts in the group here.