We have been installing screeds and polishing them for many years now with much success. Good prep is essential which all starts with a sound flat substrait for the celotex to sit on through to the miothene on the walls being adhered properly. 

Consistency of depth is important as is stapling the under floor heating pipes down with enough fixings, rising pipes in liquid screed can be disastrous especially if the screed is to be ground and polished.

Though Cemfloor is a liquid screed if you are having it ground and polished its important not to over water the mix, this can lead to issues in the surface finish so a slump of around 230 would probably be best. Its also important to use the C30 strength Cemfloor because when polishing the screed the harder it is the better the finish you will achieve.