In Brittany Pearce’s last article (here) she considered how a landlord might respond to a section 27A application where the tenant seeks to set-off part of his liability when the cost of works has increased due to inaction, in circumstances where the tenant’s non-payment is one reason for the delay. This article continues the section 27A theme by considering how a landlord might grapple with its failures to serve contractually valid demands. In a recent case, she successfully argued that, even if service charge demands were contractually invalid, the tenant was barred from arguing the point, by virtue of estoppel by convention and/or waiver.