Hi Andy
Heres my advice if you want it.
You will probably need 100 mm of good quality concrete for the hardstanding, equally important with concrete slabs is the ground beneath that is bearing the slab.. a specified job for a driveway could easily be
1.Excavate 250mm below finished surface, more if still in organic layer (topsoil)
2. Possibly geotextile membrane, however this is only nessessary if the ground can be displased with the heel of your boot!! and then with a concrete slab is is primarily there to allow stone to be compacted (wackered/rolled) without puddinging the soil below.
3. oversite 150mm of type 1 or good quality reclaimed (40mm to dust size) hardcore. grade as nessessary and compact thoroughly.
4 Set up any shuttering/ road formers etc.
5 Pour , level, screed, finish concrete with desired surface ( brush finish, tamp finish, powerfloat)
However, I find when doing what i think is a smallish job like yours I would excavate to the subsoil, if you have gone through the organic soil, are more than 4 inches deep, and this is good ground I would clean off and lay concrete directly onto subsoil, remember that when you introduce stone and compact you are merely trying to uniform the undersurface, on small jobs like this I would work at 100mm minimum for concrete thickness, wouldnt worry if the ground underneath is underlating..the concrete wont mind at all...cost of hiring wacker, buying and transporting small quantities of stone, the labour cost etc... it is more cost effective to mix additional concrete ( although in all fairness the size and conditions really depend on this).
