Someone Explain the Voting System to the Greens

Sian Berry was on the Daily Politics show today. She was asked about her having told her supporters to put Ken down as their second choice. The following is an approximate transcript (~27:50 through):

JS: At the moment aren't you in danger of splitting the left vote, the anti-Boris vote?

SB: In the first round any vote that isn't for Boris is completely safe, basically, and your second round vote counts in the final round when the run-off will be. It's always been the final round that has decided every Mayoral election so in the first round essentially you've got a free vote.

As everyone is probably aware the second round vote only counts if no candidate secures more than 50% of the first choice votes. So the claim that anyone has a free vote in the first round is simply not true.

Now, we might credit Ms Berry by supposing that she knows this but is assuming that no candidate will ever win more than 50% of the vote. This is a risky assumption given that a YouGov poll just put Boris on 47% of the vote. In answer to Jenny's question then, the answer must surely be yes. But we shouldn't complain because if it gives Boris a bigger chance of getting in all the better.