a layer of plaster will have little effect, Plaster was devised as a material that was good to decorate on to, not a soundproof system. The make up of the wall and what goes in to stud work resolves that, of course plasterboarding will help but that should have been done from downstairs flat, ie double boarding ceiling helps sound proofing and protects against spread of fire. If you are lifting floorboards the best advise would install a good soundproofing insulator like kingspan and before you put flooring down ie carpet/laminate again put a soundproof membrane down. Ask at a carpet or flooring outlet for what they would consider best.
Sorry can't be more help, I understand your frustrations.
i plastered many a floor when i was learning the trade as most of it ended up there by accident!
plaster is the most unsuitable product for using on a floor, it's not designed for it and its sound insulating properties are negligible anyway. there are materials designed for what you want to achieve and they are widely available. kbrownie has already covered that i think.
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