I think that I have discovered the explanation for what has become a common sort of grammatical error. It occurs frequently on Radio 4 and in newspapers from which we might expect higher standards of literacy.
If we use a computer grammar-check on the following two sentences, the latter is wrongly deemed to be incorrect. "One in seven members of Parliament are redheaded." "One in seven members of Parliament is redheaded."
Development is required before such checks can be used with confidence. It would be a pity if students were to become reliant upon them.
Hugh V. McLachlan, Glasgow Caledonian University.