This isn't my problem but the wife's brother-in-law, who has a house off the coast of Scotland in The Outer Hebrides.
He has serious leaks round a velux window, a built on porch at the back of the house and an added bay window. The weather tends to be wet and windy as it is on the Atlantic coast but mild because of the Gulf Stream.
Is there an easy solution he could carry out himself, if so time estimation and cost would be helpful, if not ditto with regards to somebody else doing it?