Well not the actual base cos that’s concrete, we want to carpet it so what’s best to put on the concrete before any underlay goes down? Don’t fancy putting underlay on top of concrete although I’ve seen it before.
Wayrock or similar?

Should be screeded which is normal practice and that should be enough. Then use a nice cushioned underlay.

Is the damp proof membrane below the concrete? You want to figure in time for the concrete to dry out a fair bit before laying carpet on top otherwise you will encourage mould, especially if the floor is going to be cold.

Is the damp proof membrane below the concrete? You want to figure in time for the concrete to dry out a fair bit before laying carpet on top otherwise you will encourage mould, especially if the floor is going to be cold.

I believe there’s a membrane at the very bottom, followed by some of that thick insulation then the concrete on top of that.
How long for the concrete to dry do you reckon?

The concrete should be dry before it is screeded. You can tell if the screed is dry by the colour. It will be a nice uniform light grey.

Taken from The Concrete Society website http://www.concrete.org.uk/fingertips-document.asp?id=694

The rate of drying of concrete depends on the ambient conditions in which the element exists. BS 8204-2 suggests that under good drying conditions concrete bases of 150mm thickness drying from one face will take more than one year to dry. The application of impermeable or timber flooring materials must take this long drying period into account.

1 day per mm is the general drying time although you can get away with a bit less. Even if it looks dry it may not be. If the surface is level and even then just put the underlay straight down on it, if it is a little uneven then some self levelling compound will sort it.