I thought I would check before I called anybody a liar and copied what Dottynan wrote and put it in a search engine. He did, in fact, say it in April 2001 when The Countess of Wessex's troubles in the wake of the "fake sheikh" interview prompted renewed calls for reform of the monarchy. Along with other MPs interviewed for and against the monarchy his quote was as follows and reported by the BBC:
Jeremy Corbyn, the left-wing Labour MP for Islington North, repeated his calls for an elected head of state.
"I think it's time that we just moved on and said, when the current Queen completes her reign, wouldn't that be an appropriate time to call it a day and have an elected head of state, either by both Houses of Parliament, or by popular vote."
I think his views on the monarchy are pretty well known.