A Hurwitz group is any non-trivial finite group that can be (2,3,7)-generated; that is, generated by elements $x$ and $y$ satisfying the relations $x^2 = y^3 = (xy)^7 = 1$. In this short paper a complete answer is given to a 1965 question by John Leech, showing that the centre of a Hurwitz group can be any given finite abelian group. The proof is based on a recent theorem of Lucchini, Tamburini and Wilson, which states that the special linear group ${\rm SL}_n(q)$ is a Hurwitz group for every integer $n \geqslant 287$ and every prime-power $q$.