Hi Wez, I was going to build Belle Vue Q1 and went upto the tender stage, but when they tried to charge me 7k for double glazing, i pulled out. The reason they gave me was that they will have to use raft slab type, which affects our energy rating. I spoke to many people after that, still to find a single person, who finds that reason convincing. And i made no structural changes to the house.
A word of advice, don't trust their admin staff, they are careless and rude. You are just a number for them.
Do your home work, ask questions, stand your ground, its your house after all.