I believe that the rest of the section were trained on the Bren, so if the gunner went down, one of the riflemen would take up the weapon and the no.2 would be replaced as well.
Mr. Picky says that the no.2 replaces the no.1, and you would then pick a new no. 2 from the rifle group.
In my TA days we still used Brens (L4s) as we were an internal security battalion, and almost invariably working short-handed. Even when we could produce 8-man sections, the gun group was still probably going to be just two men; same with the GPMG group when I was in the UOTC. I think the attitude was that a competent gun group didn’t need supervision from a JNCO. Mind, the gun group I remember best was composed of two Corporals, one of whom was the armourer; they knew each other and the job extremely well, and could move the Bren like greased lightning and get it into action in no time flat.
All the best,