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Kingspan (if you're talking the foam board, which I presume you are as you said 120mm) isn't a good choice for acoustic insulation, you need something designed to be used as such.
In the revamped rear of our flat we've had a load of Earthwool acoustic insulation installed between the joists:
http://www.knaufinsulation.co.uk/products/glass-mineral-wool-rolls/acoustic-rolls/earthwool-acoustic-rollThen a plasterboard ceiling which is decoupled from the joists using a resilient bar system.
We needed to buy insulation anyway (the Earthwool is also an effective thermal insulator) and it was about £500 extra to have the decoupling system.
Well worth it as I hate noise - the front of our house is original victorian and has no soundproofing or insulation at all, there's now a big difference. It doesn't stop all noise but it dampens it massively.
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Fox
Airhead
Acoustic insulation is a bitch, any material you tie to the walls/floors transmits sound to some extent. Your best chance is probably isolating the frequencies you least want and putting up with the rest. If your rooms are big enough that you can stand to put 120mm kingspan or something similar it might be enough to dampen voices.