8x7 is fairly small, someone who does it on the side would probably do it for around £100-150, but you could pay more from a full time plasterer.
If you've never done it before, or ever watched a plasterer I would not attempt it yourself. It's not too hard, but it's something that takes a reasonable amount of practice. I am not a plasterer, but my dad was and still does some on the side, which I have helped him with a few times.
If you really want to go at it yourself, make sure the walls are fairly sound. Bond with a 5:1 water:PVA mix, give it a couple of coats. Let it dry.
Just before you want to plaster you can bond again with a 3:1 water:PVA mix.
Generally for a skim, something like Thistle Multi would be fine. You mix this into water until a paste consistency (not too thick), nice & smooth. Usually you'll need about a 1/3rd of your container full with water, then add the plaster a little at a time, mixing all the time. Once to thickness, a whisk can help to get rid of any lumps you've missed.
Then, trowel the plaster on, just to get a rough coat, fairly even in thickness (few mm). Usually opposite walls would be done, but you can do adjacent walls, it just takes a little more care in the corners. As it's going off a little you will add in a little more to start levelling out and filling any bits you've missed or that need building out to get level.
As it starts to go off more you can flick some water onto it and keeping your trowel wet continue to smooth it out, until nice & flat.
Any bits that gather at edges, get a small brush with some water and brush them in.
External corners will need beading (finish) before you plaster, leaving the rounded edge as the finished corner.
This is just a quick guide on the basics. It would be an idea to practice first, or get a pro
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