Interestingly, rockwool appear to do the following batts:
Wickes stuff (400mm wide, 1200 long, 50 or 100mm thick) - 35kg/m3
RWA45 - 600 wide, 1200 long, many thicknesses - 45kg/m3
RW3 - as above, 60kg/m3
RW4 - as above, 80kg/m3
RW5 - as above, 100kg/m3
RW6 - as above, 140kg/m3
I assume that...