It’s really important that a thorough damp investigation into the root cause before specifying repairs. Whilst this particular drain defect isn’t that common, this is something I have come across previously a few times. The other issues with this property are pretty common, but unfortunately sometimes the wall appears dry internally as it’s been masked with a modern repair. Before spending hundreds or even thousands of pounds on repairs, it is best practice to eliminate and identify all the the issues that will contribute to rising damp symptoms in a property.
Whilst lime plaster is a sympathetic approach to dealing with damp issues, it is not recommended to be applied until a professional damp survey takes place. If you want a sympathetic approach to dealing with damp issues this is something I can help you with. I offer traditional repairs to include, lime plastering, lime rendering, lime repointing, lowering of high ground levels, modern paint removal. If you want a free damp survey, this isn’t something I offer. Always ensure all damp surveys follow the elimination process in BS 6576, and the damp analysis follows the the guidance in BRE DIGEST 245. Always ensure you ask for an intrusive/invasive damp survey as the non invasive damp surveys are a complete waste of time.