The Natural Philosopher wrote:
John Rumm wrote:
Lobster wrote:
Steve wrote:

If I do this, what is the best way to hold the cellotex? Builder's
tape or nailed battons come to mind, but are there are better ways
and are there any special issues related to fire regs?

You just cut it very slightly too large, and it will wedge happily
between the studs.

And fill any gaps with expanding foam - which can also be used to glue
boards in place.

and cover the studs with the special foil tape, to vapour seal
everything - and draught proof too.

Not in this case, as the plasterboard is already in place and the OP's
accessing the studs from behind.

David