In article , Geoff Pearson
writes

Second question: how to do it? I can put mineral wool rolls in the space
and hold it in place with netting. Or should it be slabs of something? Do
I need vapour barrier - I think not?

This has been done to death on uk.d-i-y over the years and the advice
given is still valid, can I suggest you have a search on google groups
group:uk.d-i-y and come back with questions on anything not covered
there.

With bills that high I assume draughts are a more likely source of your
heat loss. Floor insulation can generally be assigned a lower priority
but you usually end up fixing floor draughts at the same time. For info,
a draughty 5mm gap between floor and skirting on a good sized Scottish
lounge room is equivalent to having a window open 100mm. How draughty
are your windows? Also, tenement or proper house, it makes a difference.
--
fred
it's a ba-na-na . . . .