Attic Window needs replacing non-standard size - any advice!

I purchased a house 2 years ago. The attic was converted (without building regs, but has an indemnity policy), and the window they put in was is a non-standard size. I've looked at all the main manufacturers of attic windows, velux etc. and can't find a size that is even a near match.
Does anyone have any suggestions? Or know of a company who will make attic windows to measure? (because of the building regs issue, the fitter also needs to be fensa registered - all very complicated!) Any suggestions gratefully received!

provided you don't have to mess with any structural roof timbers, then surely you can find a size fairly close that will mean only cutting or adding a roof tile or two?

so long as the window is fitted as per instructions and has low 'e' energy compliant glass, then why fensa?

are the roof timbers fitted an unusual distance apart?

you can always fit a slightly narrower one, and pack it out. Height doesn't matter, you just have the tiles and flashing fitted to suit.

noseall said:

provided you don't have to mess with any structural roof timbers, then surely you can find a size fairly close that will mean only cutting or adding a roof tile or two?

so long as the window is fitted as per instructions and has low 'e' energy compliant glass, then why fensa?

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I thought all replacement windows either needed fensa or building control. With an indemnity policy if you invite building control onto your premises and they subsequently ask for improvements/alterations to work covered by the policy then the policy becomes invalid. Also I thought with fensa windows had to be replaced like for like size?