The buyer's solicitor is asking for indemnity for the channel out the back, under the ground, which takes the waste to the sewers, on the grounds that it has been "built over" and we do not have a "build over agreement".
But how could we possibly have a "build-over" agreement? The bit of house which goes over the sewer is original. It's Victorian. The Victorians were not known for issuing "build-over" agreements!
Do we really need one? I'm guessing perhaps they are assuming that bit of the house is an extension?