A brickie or small builder (if you can find either) would be your best bet rather than trusting something structural to a window fitter. And please, don't go to the likes of Anglian or Safestyle. I've seen the quality of some of their work, and it leaves much to be desired. That and the high pressure sales tactics used is just laughable along with the massively inflated prices.Thanks Malcolm, honestly, I don’t know what sort of window I’m looking for. I think I’ll stick to Upvc double glazing but other than that I don’t know yet.
I am tempted to get a brickie in and quote for rebuilding the jambs to consider both too. I guess I just have no idea on costs for each option. It all just sounds rather expensive!
You are going to need scaffolding for the upstairs windows which will push the cost up, but if you can get it done for £500-1000 per lintel, you're doing well. Most brickies/builders will want to use cement rather than a pure lime mortar. So if you want traditional materials used, you'll need to be upfront and firm about it.
As for FENSA and equivalent certificates - These were introduced back in 2002, so anything installed before this wouldn't have any paperwork. Kite marking of the glass is only a "thing" for toughened units which are not needed for every window.