My question is; can anyone recommend a
good quality preparation, preferably painted on to the existing plaster,
that will give me a nice smooth finish.
Or, and break it to me gently, do I have to re-plaster the room. I say
I, I mean someone else, a plasterer for instance.

You could sand the walls with sandpaper having put the sandpaper round a
block of wood so that it rubs off the tops first rather than following
irregularities. It depends on how big the uneveness is.

Rob Graham