When i say shed I actually mean a log store... It will be built out of textured white concrete block as I have a load of these for free from work, but just need to work out the base.

If I build a timber frame about 100mm high round the area, fill the bottom with crushed bricks rubble etc, and then fill to the top with concrete will this be strong enough to support the walls, Should I go deeper where the walls are going to be (along back and up each side?) I don't really want to have to shift a load of soil out as I have no where to put it, and I don't really want to hire a skip etc...

Also what strength mix for the concrete 3:1 and fairly wet?

Yes, definitely make it thicker under the walls.

Make the concrete with a mix ratio of 4:2:1 stone:sand:cement.

HTH.

John.

at 100mm depth i wouldnt put any rubble in just concrete

A 4" slab with a block wall around it will be fine.

I actually laid slabs on concrete and built mine off that in brick/ block.

No probs.

at 100mm depth i wouldnt put any rubble in just concrete
Make sure you put some of that blue PVC sheeting down as well. and either lay your 1st course of blocks on a damp proof course DPC, or use engineering bricks for the 1st course. You dont want all you wood getting wet with rising damp.

I had a mad moment yesterday and dug a foot wide ditch about 9 inches deep all round the edge under where the wall will sit, just to make sure, I was going to line the bottom of the hole with plastic before poring the rubble and concrete in as a sort of DPC...

Try and design it so you have some air circulation or logs will get damp and stay damp. DL