new tiled roof, with velux windows; ceiling plaster mouldy!

We have a new extension roof, sensibly NOT flat, with tiles. Near the outside corner of a velux window the celing plaster looks very mouldy. Most likely water leakage.

External inspection has not highlighted where the water could be coming from. All the tiles seem to be in place.

Whats the best approach to tackle this issue?
Thanks
Legolas

Remove the tiles is the only way, sometime the problem can be at the top of the roof and water running onto the roofing felt near your velux window or the velux flashing is not fitted correctly. Can you not get the builder back in which should never leak at all. What is the angle of the pitch roof?

Thanks Masona

The angle of the pitch roof is approx. 45 degrees.

We will get the builder to look this!