Window position within cavity?

[Big Jimbo]

If they are not 30mm into the cavity, they are a building control fail. However, BC will probably not even notice. If you have ever seen black mould and damp around a window reveal, it will generally be because the windows are not fitted within the cavity. This is also a cold bridge, and ultimately it will cost you more to heat your house forever. You might be able to get over it by glueing 20mm pir to the head, sides, and bottom, followed by plasterboard glued to the pir. The pir would need to be sealed with airtight tape to the window, and to the internal blockwork wall where it meets the reveal. You will only be able to do this if the frames are chunky, and can accommodate the extra build up. 

For anybody reading, if you have full fill cavity, you are not required to fit those worthless (fit after) cavity closers. If you do need to close a cavity, you would be much better off with a 50mm plus, bit of pir, and a foam gun. Hope you get it sorted. I would be asking for a copy of the written instructions that the window company received from your builders, getting Fensa involved. not paying any outstanding monies, and taking out a small claim against them.  Best of luck getting it sorted.