I am about to place a small shed in the back garden.
The garden fence is the lapped 6ft wooden panel type with concrete posts and moulded concrete bottom cross-pieces.
As the shed is going in the corner is it feasible to constuct wooden shuttering for two sides only and use the existing fence concrete cross-pieces as the other two sides for shuttering a concrete base?
If you cast direct onto the gravel boards and there is some issue with the fence at a later date guess who will get the blame? Cast the base so there is a definite gap and you should not be held liable.
"i've used old breeze blocks and various bits of broken blocks and general rubble."
If you've broken up the blocks then it should be OK but should be well compacted. How deep is this layer? The whole sub-base should be around 5 inches deep. A layer of scalpings over the top (well compacted once again) wouldn't hurt and once you've done that a blinding layer of sharp sand isn't a bad idea. You should also lay a 1200 guage polythene DPM on top. This will stop the sub-base drawing the water out of the concrete too soon and prevent moisture rising up into the fabric of the shed and its contents later on.
Concrete straight on your prepared base adding any additional layers will only give something for ants to live in ( and rats if you make it deep enough). DPM will have little effect on the dampness in a shed - I wouldn't bother unless you are going to seal the inside and live in it.