You will find B&Q and Wickes selling rolls of insulation for ?1 each.You could easily insulate the whole loft for about ?30-?40, depending on depth. The Gov says the recommended minimum depth is 270mm (11" to you and me!).This means usually 4" between the joists and the rest cross bonded across the joists. Makes it bloody difficult to see where you are treading, so be carefull. Long sleeves and a mask are good ideas. B&Q were doing a deal a while ago that they would supply and fit loft insulation cheaper than it could be brought in the shops! Now the price has dropped substantially I doubt they still hgave the same offer on. The Gov is subsidising insulation, thats why its so cheap. Good luck and don't fall through the bedroom ceiling.