I could have built my own but it would have taken a lot longer to build, so we got a good deal on a 16 x10.
Not trusting the floor they offered as being likely too thin, I elected to build the base.
Using concrete blocks on beds of gravel, we levelled the site after clearing solid 3 foot high nettles!
Then made up 10 foot long bearers of free 3x3 from Ian's work and today with Bazz's help and his frighteningly powerful De Walt battery screwdriver we laid 5 8x4s of 18mm Sterling OSB3 board and this is the result.
In the Flatlands we have a sloping site!
Having drawn out the insides with pretty much everything I want it to contain, I still have an open space of 10' x 6' in the middle. Plenty to put my Austin 7 chassis on trestles and walk all round!