joists are quite simple cross, might even be cheaper and warmer, depending what your having on floor, ie tiles for a solid floor but wooden flooring go for suspended wooden as previous stated :) send some pictures lol
like i said on the inner skin where its deep you can lay a few extra blocks beside them even on top o the old clay crap or just throw some dampish ballast cement mix just to toughen it up, good luck on your project :)
I take it your doing a cavity wall, the inner concrete blocks should be ample,you can even lay a 150 concrete inside then change to 100m above floor level,glad to be a help,could see that a few folks in here was racking there brains for a answer , good luck:)
Is it possible to spread out 3 x 8ft 100mm x 65mm plank lintels ie one on each end one in the middle and fix shed too those,then put some boarding on,must be like a swimming pool today if you had the rain what we had :)
http://www.pavingcentreonlineltd.co.uk/index.php?productID=69 ,these are the cheapest delivered anywhere, i paid £21 a metre for fossil mint its been down two years and i have not used a jet wash yet :)
look m8 why dont you spend some decent money on some decent indian sandstone and get a decent look what lasts for ever with no need for jet washing, the moss and green sh.. dissapears in the summer with the heat of the sun unlike cheap concrete crap 8)