Found a specific local defect?
Once the likely entry point has been narrowed to a genuinely localised defect, check whether the surrounding roof and deck remain sound and whether the area can be accessed safely.
If those conditions are met, see the SealForge Kit — £129.99 plus delivery for a reinforced local repair system. If the source is still uncertain, do not cover over the area and hope: continue diagnosing or use a competent roofing professional.