During debates on the Fixed-term Parliaments Bill, one or two peers insisted on saying referenda rather than referendums. I suspect the same will happen when we debate the European Union Bill. However, referendum is one of those rare gerunds for which there is no plural in Latin. I quote from footnote 1 in David Butler and Austin Ranney’s, Referendums Around the World:
‘We speak of referendums, not referenda, on the advice of the editors of the Oxford English Dictionary: “Referendum is logically preferable as a plural form meaning ballots on one issue (as a Latin gerund referendum has no plural). The Latin plural gerundive referenda, meaning ‘things to be referred’, necessarily connotes a plurality of issues.’
I once pointed out that it should be referendums to the then Lord Chancellor, Lord Falconer of Thoroton, when, in giving evidence to the Constitution Committee, he referred to referenda. The diary column of the Evening Standard reported me as correcting him. Unfortunately, they managed to get it the wrong way round!