They dined at the Healds Hall Hotel in Liversedge on Wednesday.
The Labour leader travelled to Yorkshire to show his support for Ms Leadbeater as bookies backed the Tories to take the lead in the forthcoming by-election.
The poll has been triggered by former Labour MP Tracy Brabin becoming West Yorkshire mayor last month.
Ms Leadbeater has an emotional tie to the seat as the sister of former MP Jo Cox, who was killed in the constituency in 2016, and Labour sources have been at pains to stress she is the only candidate to live within the constituency boundaries.
She previously said: “I’m going into politics to change politics, not to change who I am.”
A total of 16 candidates will contest the Batley and Spen by-election, although the poll is being seen as a straight two-way fight between Labour and the Tories.