Latest update on the glazing costs. I have my quote from Idealcombi and it's only about £200 cheaper than the quote from Norrsken, excluding installation. I've had difficulty getting all the elevation plans over to them and so I haven't got figures for the glazed gable section, so I've made an assumption on these, but I don't think I'll be too far off the mark.
Some windows are more, some are less, but overall there's little difference between the total figures. For the sake of the £200, I'm going to stick with Norrsken on the strength of the supply and fit package and no VAT reclaim, plus a nicer option on the internal frame finish. Also, the tall window in the hallway that goes to the upper ceiling needs to be done in 3 pieces by Idealcombi, whereas Norrsken can do it as 2 pieces.
Thanks for all the help and input, everyone. I'm sad that I couldn't get the prices down, but happy that I've done my due diligence on the glazing.