We're planning to put a little log cabin/shed type thing in the back.

The ground isn't particularly nice so I was planning to dig it roughly
level, place level ballast, position a sheet of concrete reinforcing
fabric/latticework on some bricks or somesuch so the lattice is 5cm above
the ballast and then pour 10cm depth of concrete to make a 3m by 3m slab
that's not going to break into a couple of pieces sinking at different rates
over time.

By my reckoning, this will be around 800 litres of concrete and, since a
cement mixer holds around 100l of mix, around 9 cement mixers full.
Neighbour has one I can borrow. However, manufacturer spec sheets say a 25kg
bag of concrete mix (the largest in keyline's web site) will cover 0.1 suare
metres at 10cm depth, meaning I need around 80 bags at a cost of around 600
quid. I can get that much concrete delivered for £118. Am I right in
thinking delivery is the cheapest option?

Whatever way I get it, I will have to work on the reinforcing fabric; moving
a wheelbarrow over it and standing on it. I'm considering some boards to
spread the load and stop me tripping over the bars. Is this the way it's
usually done or will this end up knackering the latticework? How widely
spaced will the supports have to be to stop the lattice buckling under both
my weight and the weight of a wheelbarrow full of concrete?

Am I going over the top to be thinking of reinforcing the concrete? The
fabric sheet only costs around £40 and I'm not sure what the ground is going
to do over time and under load; there's a lot of rubble under there, since I
gather some buildings were once there (used to be railway sidings and
yards).

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Dr. Craig Graham, Software Engineer
Advanced Analysis and Integration Limited, UK. http://www.aail.co.uk/