Find the actual repair boundary
On a continuous terrace, slate colour and chimney position do not always make the party line obvious.
Drainage can cross the visual boundary
A long gutter run may back up before water reaches your section.
Traditional materials are genuinely local
Chester's character studies describe simple Georgian and Victorian terraces with pitched slate roofs in areas such as Handbridge and the old port. Matching repairs should respect the existing roof rather than forcing modern components into it.