Put in the void as much insulation as possible. Also you might want to renew

the lighing cables while you are it, and install more in cae of new light

positions in the romms below. And install some 2.5mm cable ready to fit

sockets up there in case. Even some pipe ready to replace a tank, etc. You

don't want to rip it all up a few years down the line.

I couldn't agree mo Flooring over old cable would be a significant error and my first job will be to
put in some decent (and redundant in case one blows) lighting to replace the one batten holder there
at present, along with a socket or two, spurred from a handy point close by.
The (for want of a better term) internal gable end wall is adjacent to the stairs and top floor rooms so
the plan will be to put a layer of celotex over this once I have a floor I can stand on to do it.