'Smoothing' Painted Walls Before Repainting

[mjd]

I have the unenviable task of painting our hall and landing in the next few months. With the bedrooms I went at the walls with an orbital sander and managed to get a reasonably smooth finish to the walls before painting.
The hall and landing seem to be a little different though. I have no idea what the previous owner used to paint them, but it seems to be everything from brush, to pad, to roller. The finish is awful due to the mixture of painting techniques and is at its worst in those areas that see direct sunlight.

I know skimming would probably solve the problem in one go, but the cost is making me think it may be worth having a go at tidying things up myself in the first instance.
Apart from sanding, are there any quick fixes I may have overlooked?

 

[mjd]

Hehe - There isn't a hint of gloss about the place apart from the door frames I have done myself already. The last owner just smothered the frames in undercoat and left it at that. No prep at all as far as I can tell. When I stripped a few of the frames back last year there must have been 5 or 6 layers of paint on there, with the original coat being green.

Looks like I have a few good months of prep ahead