Jon,
first off you don't want it to dry straight away. Screed sets by
chemical reaction not by drying out ... screed can be walked on after 24Hrs
.... give it a soaking with a fine spray (or watering can) and cover with
plastic sheet.
Re-soak if it looks likes it's drying out .... keep doing this for first 3
days.

After which no more water .... then let it 'dry out' , it will take around a
week per 1" of thickness.

Your better half advises you are tiling on floor - then 3 weeks can be
shortened, to 2 weeks ... slight moisture will not be a problem, but would
be a big no-no if laying wooden flooring.

If it were me - I would still leave it 3 weeks.

Cheers Rick.. I think I will be leaving it for a few weeks, but
probably won't have the patience for 3 weeks (or the capacity for a
daily ear bending from Helen )

I am glad that I can walk on it after 24 hours, as I have loads of
things I need to do (specifically the electrics) and want to get
started on SOMETHING. This whole project has takens 3 times longer
than I planned, mainly due to the planning permission that I found I
needed at the start. I am now just anxious to get busy on the things
that I can do. The builders have taken weeks to do things that should
have taken days. They were ment to be coming on Saturday to 'finish
off', but now its been postponed (again) to Monday.. But the end is in
sight and hopefully, this time next week, I will be able to get
going...

Thanks for the advice from everyone.. All the best

Jon