Stu - Yes, I've just done this - last summer.
Mine is 14.6m2 and 2.4m high.
I cast a concrete base, then built the walls from 7nm blocks, it is lined in 24mm celotex, covered in 11mm SBS board.
The roof is packed with celotex, and loft insulation between the joists, and clad in Galvanised steel sheet.
It has 1 x small double glazed unit, and a white, powder coated steel security door from ebay.
I've not finished the electrics yet, but so far it has cost just over 3k - I did everything myself, except laying the blocks, which cost me £400..
We costed up good quality wooden sheds, but decided this offered better value for money, as it could also work really well as a garden office, or studio.