After a careful survey of the roof we located the problem areas and repaired them. In the photograph, taken just after the work was carried out, you can clearly see the lighter areas where we patched with new material. We also relined the guttering with a type of corrugated sheeting which comes in a single strip. Most leaks occur where materials join so this reduces the chance of leaks straight away.
They still haven’t asked us to refurbish their roof because our original repairs are still holding. Obviously, since their roof is sound and leak-free we have advised them not to.