[quote author=Lee_Elms link=1098704902/0#2 date=1098706863]Thanks for the prompt reply.
Is 12.5mm to be preferred over 9.5mm for ceiling usage because it is stiffer and therefore will sag less between fixings?
-- Lee
[/quote]
you have answered your own question
as for the other, regulations state that you need 1/2hour fire resistance. part of that is having 2mm of skim on the plasterboard
because you just want to tape & join the fireline may give you the extra bit that you should have.
i am not quoting regs here, but if you need to know more i/we should be loking at the british gypsum site.