Concrete slab for garage - how to do

I've got an existing slab there currently. Quite old and uncracked. I'm going to cut into it to build the foundations, but one end is 11cm lower than the other, so I need to pour some concrete to level it up

The garage is 4.6 by 6m so about 2.5m3 of concrete needed.

1. Can I use the same mix of concrete for this new overslab base as for the foundations (C20) and hence pour all in one go?
2. I'm tempted to put down a Damp Proof Membrane before pouring this new concrete with the aim of reducing future damp from teh floor. Is this possible for such a thin floor?
3. Apart from covering the concrete with plastic sheet to slow the drying, is there anything I should do to reduce teh chances fo this new slab cracking when it dries?

You can but a DPM under the new pour, to help the concrete strengthen, it is a good idea to cover over to reduce the hydration period, it is also worth uncovering the follow day, after the pour and watering the base down. Leaving the base covered in water if possible.
What are the foundation for and do you mean footings?

The problem you've got, is where the new slab tapers towards the higher section, the thinner it gets the weaker it will be. Concrete doesn't stick particularly well to itself, so it will be liable to cracking and breaking up if you drive onto it.

If you can lay it thicker, say 75mm thickness at the highest point, you might get away with it, but this would mean raising the floor and rest of the garage to compensate (not to mention more concrete needed) plus you would have to build a concrete ramp of sorts in order to drive in.

The best job would be to hire a breaker to smash it up as small as poss, then use it as hardcore under a new slab of at least 100mm, with 6mm (A142) steel mesh added as a bonus.

To answer your questions:

1) Pouring them seperate will be much easier, unless you've got loads of helpers.

2) Good idea, but it must lap up to meet the wall dpc, it won't unless less than about 75mm of concrete though, as stated. A bitumen liquid dpm might be a better option, 3 coats brushed on with a broom.

3) You can add fibres to the mix, these can be bought from builders merchants and will reduce any cracking somewhat. The poly sheet is a must though, left on for a couple of weeks if poss.