shed base dry mix

I've levelled an area of garden ready for a 6x4 shed which I intend to set onto paving slabs. The slabs will be set onto a 2 inch dry mix of sand & cement.

What type of sand should I use though ?

Also, how much is in a "jumbo bag" like =ttp://www.wickes.co.uk/Aggregate-Sand+Ballast/Building-Sand/invt/220080]this one

You could omit the cement and just use sharp sand to level off the ground before putting the slabs down. The most important thing is to elevate the shed from whatever you put it on, as moisture will still soak into the shed base when the slabs get rained on. I'd suggest using treated 3x2 or 4x2 (6 metres should do it) with a strip of thick polythene, dpc, or shed felt on top of each one.