First each party must select their candidate. Different parties have different mechanisms but most seem to come down to a vote of members in the borough.
In May 2018 Lewisham voters will get two votes; a 1st and 2nd preference for their choice of mayor. A candidate needs 50% of the votes (plus 1) to win. If no candidate gets 50% in the first round all but two candidates are eliminated and the 2nd preference votes are redistributed to the top two candidates.
We had a referendum in 2001 on a directly elected mayor system.
Residents voted overwhelmingly (51.4%) to have a directly elected mayor. Steve Bullock was elected in 2002 and has remained in post for the last 15 years.
Apparently he won all three rounds, it wasn’t even close, which at least means he has good backing among the party. As he is almost certain to be our new Mayor and he’ll need all the help he can get.