CW if you are going to try this yourself and your skills are less than perfect then an easy method is to;
give it a coat of plaster, shining a light along the wall to show up any
irregularities and keeping it below the original surface.
Let that go off fully then,
as it is only a small area,
mix up a box of Polyfilla (don't get the stuff from Screwfix its carρ) and trowel that on.
It goes on a lot easier than multifinish, will feather in to the old plaster easily and sands down without the need for an industrial machine
You should be able to get a perfect finish this way.
There are finishes available in a plastic tub, expensive and the one I tried was absolutely rubbish.