Once the plastic has gone on, the builder is coming back to lay approx
80mm of screed onto of the concrete that he laid a month ago.

Is there any insulation in this floor? There should be large chunks of
Jablite or similar going down, as well as the plastic sheet (DPM). Although
not compulsory in many conservatories, it should still be done for your own
comfort.

If I need to wait, is there anything that I can do to speed up the
'drying' process (i.e a de-humidifier)?


Concrete doesn't "dry". It sets. It actually needs water to set properly.
Removing the water by overdrying will actually slow down the setting process
(sometimes so much that it never completes) and damage the floor.

Basically, you should wait the recommended time scales. How does the phrase
go? Bodge in haste, repent as leisure. Or something like that. ;-)

Your floor will take much longer to complete if you disturb it too soon,
cause gaping cracks and need the whole lot ripping up again.

Christian.