Leaking roof. Is the skew at fault?

First post here so hopefully I can describe my problem. I have a 1899 Scottish Cottage with an extension constructed in the 1960's. I have a leak in the gable end of the extension.



There has been pieces of roofing felt slid under the slates to try and solve it in the past but the leak is still coming. My question is looking at the state of the skew and mortar fillet (been sealed before) is it to blame and how much would a replacement of fillet and pointing between the skews themselves? Skew length is about 6m.



I can send a high res picture of the skew to an email address if that would help.

Thanks in advance.

i it looks to me like there is no lead underneath the copping stones on gable end i i would reccomend taking of the copping stones and put lead underneath or make sure the slates overhang gable end by approx 50mm then put copping stones back on my bet is the slates dont have any overhang cost would depend on what route you took extending slates ove gable would be more expensive but probably most effective