Alex Dady wrote:

What's the cheapest way to sort this out.

Cheapest way is to do nothing, but won't achieve the desired result. You
really need to spend /some/ money on insulating it.


I have been told that
cardboard makes a reasonable choice

By whom?!? That is a really stupid idea, for lots of reasons.


but I guess it doesn't look good
and is certainly a fire risk.

And isn't a good insulator, and will absorb moisture.



I don't want to spend much money as the
house isn't mine...

But you want a useable space, right?


Would some insulation material and a load of plasterboard do the job?

Yes, that's what you need - at least 40mm of insulation board, then
plasterboard. The ceapest insulation (polystyrene) can be had for around
£6/sheet. Plasterboard costs a similar amount.


--
Grunff