How long does it take?
As long as it takes. No need to rush or you're more likely to inflict damage. Go slow and use lots of lube.
Is it difficult?
No. It's one of the first things I started doing after I read about it on DW and I was a complete noob back then.
Do I need to polish and wax straight after of can I leave it 24 hours or so (going to try on a night when my daughters gone to sleep)
The problem is that you're removing contamination. If you leave the car, you're more likely to get re-contamination, undoing your hard work. I would honestly set aside a weekend if you're new to detailing. As above, if it's garaged in those 24 hours, less likely to be a problem but I would still wash before polishing.
Any recommendations preferably from halfords cause I can get some tonight and get cracking.
In all honesty, I loved the smooth surface clay kit from halfrauds. It was easy to use and the lube was great but didn't last long - I used 3/4 bottle every time I clayed.
Probably a daft question but will any QD work with the kit?
Different QD have different properties and so have different amounts of lubricity. I used to use Megs Last Touch but just didn't get on with it and now use Dodo juice born slippy. I use this even with Bilt Hamber clays which use water as lube.
Also how long will the kit last
As others have said. You get two bars. Chop them in to half so you have four pieces. If the car is older and never been clayed, you may well require more than one piece. Remember to warm the clay in your hands so it's as soft as blu-tack. Do a section and then inspect the surface. Fold in half when it looks dirty. Keep going. If after several folds, there are no new clean surfaces of clay to use, chuck it and use another piece. If you drop it, chuck it and use another piece.