concrete slab laying

That depends.

Clay that is now covered by a slab may well dry out and shrink, cracking the slab and it also may not stay level.

Uneven clay uses up more concrete.

In general the use of some sort of vaguely moveable layer underneath - sand ballast or hardcore - helps isolate the slab from ground movement..and that means you can use less concrete.

If you want to save labour at the expense of materials, just level it all off with some ballast or even sand, pour a bit of concrete, lay in some reinforcement - chicken wire or rebar mesh - and pour the rest - up i'd suggest about 4" (100mm)..and level it all off.

If its not going to take much weight, and is not a huge area, you can skip the rebar etc.

Ta. Food for thought

Food for thought. Thanks for that

The stuff in this video is clearly not Osprey proof, but it would probably be fine for light machinery. It might be a lot less work than concrete.

The OP hasn't said how long he wants this to last, or whether he minds if it cracks up a bit. What you've described is a proper job, but maybe he doesn't need that?

Well if it outlasts me (77yrs old) I'll be happy

OTOH, it's good that you're fit enough at age 77 to do this work.

Helipad in this bbc video means a field with a bit of loose plastic thrown around the bit of grass the helicopter lands on.

The sort of plastic used to turn a field into a car park.

These people had a helicopter proof patio

Well yes, we all know how strong a concrete block is, so have an innate feel for how string concrete is. many slabs fail because they are more like screed then concrete - far too little cement.

My feeling is that a DIY job ought to be done without skimping materials, that are in any case not hugely expensive

Ive got a pad that used to have a shed on it formed from paving slabs only - thats getting on for 30 years old and is still fine

One of the cheapest ways to make a decent surface for garden machinery BUT you need to level the clay...

If the ground is ok, then you could just lay down a vapour membrane and pour direct onto that. However for chapter and verse:

No. It just displaces a bit of concrete that otherwise costs. There are other ways to do shed bases too, you don't need concrete to store light machinery.