On Fri, 17 Feb 2006 12:31:41 -0000, "Ian_m" wrote:


The back of it says "British Gypsum 2400 x 1200 x 15mm" and looks nothing
special, no foil etc etc


Maybe regulations have moved on since your house was built.

25mm ( 2 layers of 12.5) of plasterboard gives 30 minutes fire barrier
and much better sound barrier than just one layer. But all that
progress goes to Hell if you put in downlighters without anything else
behind them.