Leakyroof

For more than a year now, my partner and I have been trying to pinpoint the root cause of a leak on our roof and have roof works done, but in vain. The roof is still leaking. Our roofers have confirmed and done the following: - Coping stones are less than 10 year old (unlikely to be porous) - Coping stones have dpc underneath them (protection from water ingress if coping stones were failing) - Replaced the lead flashing (to correct position if lead flashing had moved/slid and caused the leak) - Re-rendered the parapet wall (to avoid having any cracks in the render that might have caused the leak) - Repointed the joint between the coping stones (to avoid water seeping through between the coping stones) Need confirmation: - dpc under the coping stones has been applied, but probably not unders the last coping stone (old sandstone keystone at the front) - water test (buckets / watering cans) splashing onto the coping stones - no results - water test (buckets / watering cans) splashing onto the tiles / flashing - water ingress wall, but slightly further up the party wall, NOT where the usual leak is occuring Potential root cause > proposed solutions by our roofer: - last coping stone failing (block keystone, sandstone) > cap the coping stones with torch / felt system - head lap coverage of each tile / lead soaker issue > front elevation needs to be redone at the correct batten spacing to increase tile head lap Other potential root cause: - cracks / missing render on parapet wall (neighbours' side) allowing water to penetrate / run through the coping stones and appear on our side? > (roofer's answer: highly unlikely to cause an issue to your property unless they have the same issue in the same place) How would you resolve this issue?