This might help ,the glazing regs. ,unless things have changed since last year .
25 percent , plus the area of doors and windows that you are covering up.
Clive12345 - thanks for pointing this out, I missed that. The extra area provided by the doors and windows I am covering up would help of course and equates to a further approx 5 sq m in my case, so up to 9 sq m in total. Enough for the bifold doors and one other window of 1 sq m. but not for the roof lantern.
I had a local architect come around yesterday and he suggests that I submit a "pre-application" application form to the LABC Dept (which is the closest we can get to a chat with our BCOs here) and see what response I get. It doesn't cost anything though it does require quite a lot of detail in the way of photos, sketches, etc. I'll have a go.