Adding btinternet email to Mail on Mac

I've been using my btinternet email account via Mail on Mac for 20 years. It stopped working this week. Not the first time this has happened. As previously, I deleted the account, planning to add it back manually. However, I find I am no longer able to. I cant get past the automatic "add email" dialog - it just returns an "invalid" message and no longer (as Mail on Mac used to do...) take me to the advanced server settings dailog where I can tweak the ports numbers as btinternet requires. What happened..?