I have a piece of marble that was polished but someone has tried to clean it with an abrasive cloth. It is white/grey flat and about 8 inch dia. Can anyone suggest how to regain the polished condition?
As a trial-and-error DIY exercise you could try toothpaste with a soft cloth. If it works on a limited area then decide whether to do the whole thing, which will take quite a bit of effort.
I used to do it for a living as a marble mason, without the the required equipment you will find it very difficult. It would normally be polished using oxalic acid and pot'e powder or a high speed buffer and buffing wax.