Dave,
Count me in. I am expensive though. It will cost a bacon, egg and sausage bap with brown sauce. No snot on the egg but the yolk has to be runny []
Depending on the finish you want on the concrete you can get them to add some extra water before they drop it. This will give extra time before it goes off. You can also add water yourself and mix it in which helps.
As mentioned you will need a screed sturdy enough for the job. A good item to have to hand as well is a large sheet(s) of ply and some visqueen as if things go wrong like punctures on the barrows, the mini digger breaks down etc etc you can always dump the stuff onto the ply and visqueen to save you being charged extra by the readymix people.
How many Metres are you getting Saturday? How deep do you intend the concrete to be? Do you have a vibrator? [:$] One of these is used after the pour to remove all the air from the concrete giving you a really strong finish. Not necessary if its only an inch or so but if you are going thicker I would consider hiring one. They look just like dildos connected to a small engine. You could borrow my Wifes but if I am not there she will be using it Saturday morning []