The good news is that the treated glass remained completely clear whilst the rest fogged up immediately. In fact the area around the tax disc holder where I missed some bits also misted up. I shall now do the rest and look forward to clear vision. I bought the stuff about 10 years ago for a previous Minor (HOB 380D) and I can't remember how long it lasts but this is certainly worth doing.
Picture of HOB. A cracking little car.[frame]