JWBH wrote:

thanks to all. i've had a look at the screwfix ones and they look
the same as the argos ones. screwfix do a pdf erection guide, which
shows it going on to a concrete slab with no integral flooring. which
makes the whole business a lot more difficult for a novice.

You can get concrete ready mixed and delivered for around £90 per m3, you'll
need 1m3 to cover an area 100mm thick and 10m2, that is to say, put up
shuttering of any old timber in a 3.1 X 3.4 shape, put a sheet of polythene
in the bottom and order the concrete, barrow it in and level it up with a
plank, then give it a wipe over with a trowel, you'll have an hour or so
before it becomes unworkable, it doesn't matter if it's a bit rough and if
it rains you get a mottled concrete floor at no extra cost....it doesn't
matter if you only do it 75mm thick or whatever you like.