Chris,
Dunno whether you're still deciding (if not this might help someone
else) but I've just built a patio out of these things and I can quite
confidently say, you really don't have to think about screwing them
down - they won't move!

Having said that, if your steps aren't going to be the full 8.5 feet
of sleepers then you may have to stake them or put some sort of
concrete foundation.

You might also want to get some weed-proof cloth. I can't remember the
name but we put this underneath the wood to discourage weed growth in
the cracks.

Another hint: invite some friends over to lift the sleepers. We had 20
to lift up a sloping garden and it took us hours and knackered our
wheelbarrow - these things are heavy!!!

We got our sleepers from www.railway-sleepers.com/. A very nice chap
called Richard delivered them at short notice and exactly when he said
he would! Great service (and no I'm not connected with him in any way
etc etc).

Good luck,
Ben