Just wanted to update this thread to say that we got two of our four rooms done with calibal boards and we've noticed a huge difference.
My husband did all the work himself, there was absolutely no insulation in our house so he stuffed the bottom and tops of the boards (and all around the window frames) with rockwool and used an acoustic adhesive/silicone filler as well.
Voices have dramatically reduced, where we used to be woken half the night with noise I can honestly say I'm having great nights sleep
Anything we do hear is much more muffled and rather than sounding like the neighbours are living with us they now sound a million miles away.
We are going to get the other two rooms done asap and just need to get it all plastered and re-decorated then.
I would recommend this stuff to anyone else who is considering soundproofing, its definately worth the investment.
Edited to add we used the thickest boards we could get for most of the walls but had to use the thinest boards on our bay window because of of tight space at the window frame.