It used to be the case but now you can full fill the cavity , we was using 100mm as that's what cavity we had and it was so big we couldn't get it in the wall so changed it for 75mm and as you can see it easily fills a 100 mm void.
Timbers and roof tiles finally arrived today so Jeff can crack on . I've been told that I need to choose the fascia board but can't make my mind up wether to go white or black or anthercite on fascias and door.

Cheers paul