Look at the pattern
Condensation often affects a broad area; a leak is more likely to follow one route.
Check extract ducts
A bathroom duct ending in the loft can add a surprising amount of moisture.
Recheck in dry weather
Moisture without rain moves condensation higher on the list.
Want a roofer to look at it?
Describe what you can see and the weather in which the problem appears. That gives the roofer a useful starting point.
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