What mortar mix for laying slate fireplace hearth? - And what consistency of mortar to use?

[NovicePaver]

Sorry if this is off topic but I thought you'd be able to help.

I want to lay a slate hearth on an existing concrete base, which at the moment is flush with the surrounding floorboards.
I plan to chisel away the top 10 - 20mm, refill with fresh mortar, level it with the surrounding floorboards and lay the hearth on top.
Does this seem reasonable?
I'm not sure what ratio of sand and cement to use and whether it should be dry or runny and should I add PVA?
Presumably it should be quite weak in case it ever has to be removed?
I've laid paving slabs with an almost dry mix and thought this would be the best approach but not sure?
Thanks, John

[matt h]

If you intend to use the fireplace i believe the hearth should actually be 50mm above finished floor level nowadays. It definately does if you are going to put a gas fire there. see mainsite ref mix