Water leak from window causing damp, mould in loft stud wall

[LondonW5]

Hello Forum,

I need some advice. I have a 2 year old loft extension and during the recent heavy rains noticed a small puddle of water appeared on the floor of the lofts bathroom floor below the window. I think I have found the source of the water leak. The aluminium sash window is sat on lead that is dressed underneath and down the externally clad manmade slate wall. Water is getting into the sash window which has a drainage gully the should direct the water to the outside via a drainage hole. But the water is travelling inside the frame to the edge of the lead and going into the stud wall. I have a 5 year warranty from my builder.

So obviously the window needs to be taken out and the leadwork examined and repaired / replaced. The window also needs to be examined to see if this needs replacing. My question related to the damage the water had done, as this leak has been occurring for the last two years without my knowledge. The stud wall has manmade slates on the outside attached to battens, then there is OSB board clad with a membrane outside. Studwork wall is C24 timbers with 90mm Ecotherm between the studs. On the inner face is another 25mm of Ecotherm and 18mm WP ply on the inner face which has been tiled.

As it happens I am in the process of refurbishing the family bathroom on the 1st floor which is beneath the loft bathroom. I was able to get up to the underside of where the leak was seen and the wood of the inner 18mm WP ply has black mould and if I push a screwdriver in I can get an inch in as its soft and rotten. Also the 18mm WP Ply which is on the bathroom floor is also has black mould.

The builder should put it right, otherwise I have my house building insurance to fall back on. This is what I think needs to happen :-

1 - Take out the window and fix the source of the leak, ie repair the lead flashing and maybe replace the window.
2 - The Tiles on the bathroom wall need to be removed under the the inner skin of WP ply exposed to determine how far the water and rot extends internally.
3 - The outer slates and membrane should be removed to determine if the outer OSB is rotten and if so its replaced
4 - Do you think the 18mm WP Ply on the bathroom floor needs to be replaced where the leak was or could it be treated with a chemical wood treatment ?

Any advice on how I should tell the builder what needs to happen ? Or should I just engage my building insurance and have them fix it or have them deal with my builder ?

Any advice much appreciated