Ceiling Damp around Velux Window but no Evidence under Tiles

[AdrianDraper]

We have discovered a patch of damp (which grows during and after heavy rain) around the top of a Velux window...but having had some of the tiles removed, there is no evidence of rain seeping in, or damage to the felt etc. Everything seems fine. And repositioning the tiles hasn't helped. Even if we were to remove the ridging tiles, there is no evidence of water leaking down to the particular spot. It is baffling.

[welsh brickie]

it could be down to a cold spot, usually due to the insulation being short around the velux, this can cause a damp area to show on the plaster

[AdrianDraper]

Thanks - and this would appear to get larger as the rain poured?