The case of Peter Cain v Mayor and Burgesses of the London Borough of Islington  shows tenants wishing to challenge service charges should say so when paying those charges. This is to avoid the inference that by paying such charges they are agreeing or admitting 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. This is an important point because a tenant who withholds payment of service charges because it disagrees with them may find the lease can be forfeited by the landlord. Therefore, payment under protest is often their only option, pending bringing legal proceedings to challenge the charges.