Richard wrote:

As someone else pointed out, I dont really need soundproofing come to
think of it - just the physical barrier for security and the
fireproofing, so I guess fireproof plasterboard on both sides of a
single skin stud would be effective.

I wouldn't... you say security is a concern: if you consider that
there's a risk of a scrote breaking into your neighbour's house and
climbing up into his roofspace in order to enter your house, do you
think a couple of sheets of plasterboard will impede his progress for
more than a few seconds?!

Also, I don't know how big your loft hatch is but that might well be an
issue when it comes to taking sheets of plasterboard up there; at least
you're going to be restricted to using small bits only.

David