I would use 6 concrete blocks, the sort used for building 100mm walls.
About £1 each.
Place these flat side, 3 along each side of the propsed shed position with
the end ones in a little from the corner so you don't see too much of
them. Cut them half into the ground, and all level to each other.
Then place 3, 3inch square posts across the blocks as bearers.
You might like to put a piece of roof felt between block and bearer. Erect
shed.
mark
I don't know these concrete blocks? where to get them? they better than
4 inch x 4 inch wooden bearers?
Ed
Blocks arn't instead of the wooden bearers, they support the bearers and
keep them off the ground.
94p each at Wickes:
http://www.wickes.co.uk/Dense-Blocks...ck/invt/154040
3" by 3" posts would be adequate for such a short span, which would be the
width of your shed, 3foot less block dimension.
mark