My ceiling was starting to fall so I pulled it down and need to replace it, but am not sure what to use. I removed horse hair plaster which was up to an inch/25mm thick and want the equivalent or better sound resistance. Can anyone recommend the appropriate thickness of plasterboard and/or should I use insulated board (I'm not that bothered about heat, but wouldn't mind if it insulates a bit better)?
Jeff, the modern way, is using plasterboard.
If the ceiling is upstairs with a loft,attic or flat roof above, then you need to think about insulation.
The best insulation is polystyrene, buy it in sheets, no more than two inches thick (easier to handle and cut) cut to fit between the joists, make it a tight fit.
When you have filled the spaces between the joists, then add another layer of three inch thick polystyrene tightly butted, no gaps or holes below the joists, then fix the plasterboard below the polystyrene.
This will make your room nice and warm and cheap to heat, the polystyrene and plasterboard will pay for itself in a year or so, with the heat you will save.
If its a down stairs room, with another room above, just fit the three inches of polystyrene below the joists and the plasterboard.