Rigid fibreglass is really for US folks, over here we use rockwool or rocksil slabs, usually 1200mm or 1000m x 600mm, and in different thicknesses but don't go less than 50mm. 100mm is preferred for bass traps. I think it's RW5 or RW45, what you should look out for. Available from Travis Perkins and all good builders merchants.
