Filling floor recess to match tile level (pics)

[outmyway]

I need to fill this gap to match the level of the tiles so we can carpet over the top.

Its approx 50mm deep and 500mm wide by 7.2m
(0.19 cubic meter or 3.8 square meters by 50mm deep)

I've thought about screeding but could I fill with say pea gravel and use floor leveller over the top?

Or just use something like this deep base leveller?
http://www.wickes.co.uk/Setcrete-Deep-Base-Floor-Levelling-Compound-20kg/p/133780

 

[vinn]

Use a mix of 3:1 or 4:1 sand and cement.
Slurry the floor with 1:1 sand and cement in a soupy mix.