First and easiest course of action is to ensure that the "gaps" between the metal flashing's surrounding the window and the adjacent tiles [or slates] are not "bunged up with" dirt/ dust/ debris.
As an aside a few images of the externals will probably assist?? if possible please??
Hi and sorry for the delay. Not been to the house but have received 2 photos. Not ideal but give an idea of location of the problem.
The wood, internal and external, holding the glazing in place, has rotted so needs replacing.
Is this a job for a joiner? A glazier? A skylight window specialist?
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