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Airhead
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-roll
Then 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.