DerbyBorn wrote:

We have a window at the top of our stairs. We have never opened it.

I bet you'd have a damned good go at opening it if it was your only
means of escape in a fire! Does it open onto e.g. a flat roof, do any
other windows?

I am wondering if I could just make it a fixed window as this would be
cheaper and would give a bigger area of glass.

http://www.planningportal.gov.uk/per...s/doorswindows

"When replacing any window, the opening should be sized to provide at
least the same potential for escape as the window it replaces."

That does say "should" rather than "must" ...