On Friday, 17 May 2019 09:41:23 UTC+1, Darren Chapman wrote:
From what the surveyor has said (I've not got the report yet) there is a
small part of the wall missing (never built). Is this a case of building
a lightweight concrete block wall up to roof level? I think it's only
a metre or so long at one end, so tapers to nothing at the eaves.

Block (laid flat, rather than two leaves) will be easier to do from one side only, or can be done in stud and plasterboard - possibly double-clad each side.

Does it need sealing in anyway to the roof? Can't see how that would work,
and there doesn't seem to be any sealing for the rest of it.

Fire-rated intumescent foam.

Internet is inclusive. Survey report might have more info I guess, but
I'm not holding my breath!
Any advice?

Building Regs may apply if it's forming a new fire barrier, so factor in the cost and time of going through that process. Also Party Wall Act :-)

Owain