I cannot understand why the Polyfilla did not dry, it normally dries quite quickly, is the wall damp?
Was it an oil based gloss paint that is peeling off. It was quite popular in hallways a few years ago.
If the walls were replastered recently, the plaster is presumably sound.
Rather than sanding lightly with a sander, I would consider rubbing it down with wet and dry paper, better still those foam pads covered on both sides with abrasive would be ideal for the job, when dampened, would make far less mess. These can be obtained quite cheaply at a discount warehouse in Hugglescote, which is quite near to you. Not sure if I can mention the name of the place here.
I would also think that the best paint for the job would be a good quality matt emulsion.
You could also get some good advice from the decorating adviser, in B & Q, there is a store in Ascot Drive, or in Loughborough.