The recent case of Peter Cain v Mayor and Burgesses of the London Borough of Islington  shows that if tenants dispute service charges, they should indicate this at the time, even if they do pay them. Otherwise, payments could amount to an agreement or admission that the charges are valid.
However, if more than one payment was made, and the tenant did not say (or otherwise indicate):
that he disagreed with the charges, or
that his payment of the charges was not an admission that they were reasonable
then those payments could amount to an agreement or admission that the charges are valid.