You wouldn't get anywhere with an action for libel because the comment would be unlikely to lower your esteem in the eyes of others. (i.e. no sensible person would attribute any credence to it. Such a comment would be far more likely to prove to others what an utter prat the poster is).
However, if you want some suitable terms to threaten him with, try something like this:
"I would draw your attention to the provisions of the Libel Act 1843 (as amended by the Law of Libel Amendment Act 1888 and subsequent legislation, including the Defamation Act 1996) and the Defamation Act 1952 (as amended and supplemented by the Defamation Act 1996). Should no offer to make amends for your scurrilous allegation be made (in accordance with the provisions of Section 2 of the Defamation Act 1996), it is my intention to seek civil damages from you through court action".
It's all utterly meaningless (except that the legislation quoted does exist) because:
(a) you can't sue somebody if you don't know who he is ;
(b) any action would fail for the reason stated above ; and
(c) you can't get legal aid for a libel action. Your doomed action could cost you 'tens of thousands' (or possibly 'hundreds of thousands').
But it's what you asked for ;-)
Chris