The style is normally reserved for younger sons of hereditary peers, as they cannot be addressed as Lord, whilst daughters can be addressed as Lady. This is in contrast to Right Honourable, which is reserved for members of the Privy Council, commonesrs and nobility alike.
Surely, QM, there can't be a number of lords simultaneously occupying a single office of mayor or provost? But there can be a number of mayors or provosts, each of whom is a lord. So I'd go for Lord Mayors and Lord Provosts. Isn't the English language fun!! ;-)
Fun, indeed, H, as you say. In a colloquial/informal setting such as AnswerBank, I can see nothing wrong with writing, say, "The Lord Provosts of both Edinburgh and Glasgow attended the ceremony." However, I wouldn't be at all surprised to see - in The Court Circular or The London Gazette - "The Lords Provost of both Edinburgh and Glasgow attended the ceremony."
Have to disagree. I go for Lord Provosts and would consider Lords Provost to be incorrect. My reason is that Provost is a noun, not an adjective, such as in Lords Spiritual and Lords Temporal. You would say the Lord Archbishops of Canterbury and York, not Lords Archbishop, which sounds absurd. Here Archbishop, like Provost, is the primary noun, and as such should be pluralised, whereas Lord is used adjectivally.
I only said, "If you prefer," Mike. Many people prefer things that are incorrect! The fact is that the title IS applied to people other than Privy Counsellors, so we both got something somewhat wrong.