When a repair makes sense
If the problem is limited to one area — a few slipped tiles, a damaged ridge or a failed flashing — a repair is usually the sensible option.
A good roofer will tell you if a repair will do the job rather than pushing you towards a new roof.
When replacement is the better option
When tiles or slates are failing across the roof, the fixings have corroded, the felt underneath has rotted, or you're paying for repairs again and again, replacement often works out better value in the long run.
Questions to ask
What exactly is wrong? Can you show me photos? What's the cost of a repair versus a replacement? What materials would you use? Get the answers in a written quote.