Thanks for your comments guys, now to confront the roofer and judging by his response when I 1st questioned this and other issues it's not going to be easy. Hopefully building control will be able to back me up as well.
I'm after some advice...
The roofers have just finished and have installed 3 roof windows along with a completely new roof as part of what has turned into a nightmare of a loft conversion, but that's another story.
I am under the impression that the tiles running down either side of the window...