In message of Fri, 4 Jul 2008, Pete Stockdale writes
Best solution is to dig a six inch deep by 9 inch wide trench foundation
underneath the base .
Dig similar 2ft wide trench similar to act as footway between ends.
This gives you a sort of 8 shaped concrete in plan
Pour barrow/ready mix with security fixings embedded therein, with assembled
base thereon.

Leaves plenty of space for borders therein.

Thanks everyone for your replies. I decided against the concrete slab in
the end. It's an old greenhouse (can't afford a new one right now) and
although things may improve when it is moved, it does let in water here
and there.

So I asked the builder to do just a 9 inch foundation, haven't done a
central path. I don't want to grow stuff in soil in the greenhouse, and
so underneath the concrete foundation and covering the whole area is a
4-inch layer of gravel/grit. On top of this I am planning to put a
semi-permeable membrane, a central path made of paving slabs, and fill
up the rest with some bigger-sized gravel. I am hoping this will give me
a good balance of humidity and drainage.

Hope it works!

Thanks again for your responses, helped make up my mind for me.

Colette
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Colette A. O'Brien