Need more information. Sound blocking is all nothing. It's a bit like a water leak as you either stop it or it keeps coming. Shove sound acoustic wool in and diy sell slabs of that but can't say if it will do much. I've always been disappointed with the results so moved on to full sound proofing
The roof is a flat shared roof with my next door. Original roof used to be split but previous owner and neighbour had it joined. However this means sound travels easily above the plaster board ceiling. When you say full sound proofing what do you mean by this?
Applying sound block plasterboard without resilient bars all the alternatives your just wasting your time. Do it properly or don't bother. All or nothing. Then really you need to do walls but you can do the ceiling first and see if that's enough. It's not cheap to do though.