reccomend a self levelling screed for concrete floor

It isn't self levelling.

It is self smoothing though.

By best thought would be - if I did it again - to remove any scrappy stuff, and then get some thin battens - 1/4" maybe, and some bits of ply to pack it up with, and lay a grid of strips on the floor all dead level with a spirit level to true it up.

THEN use your compound, scarping it off thee strip tops with a piece of scrap batten. Float it even.

When set, remove battens and fill in the 'ditches'

Unless you have a damp problem, you really don't need a latex screed, which is expensive and not that easy to use. I've used pre-mixed general purpose mortar with 4 water: 1 pva for this and it holds well at the edges. Getting the floor smooth is pretty straightforward with a length of timber. Getting it level is not so easy and can give you problems at doorways etc

I did something like this using B&Q self levelling compound, which appeared to be a rebrand of the Evostick product.

I then laid 33cm square tiles.