G'DAY DAVID,
i've been using swags for years. the one i have now is similar to that which slunnie described. it is a double with a permanent open vent at the feet, a full height vent behind your head that has a zip up flap so the draught can be regulated. the top entry has a canvas flap that zips right down to your knees with a similar length fly screen under it.
with a good load of bedding you will never freeze, it takes about 2 min to set up, is water proof.
you do get condensation inside from your breath, but this can be minimised by allowing air to citrculate from the head vent to the top opening (if not raining).
we bought it about 9 yrs ago and still going strong as the day we bought it. it has those plastic quick clips that slunnie is scared of, but they are fine. it cost us about $550 at the time and it was the best we could find on the market.
the only 2 problems we have are that 1, my wife has nowhere to get dressed and 2, if it is raining when you go to bed or get up, you have to open the top to get out, allowing buckets of rain to get in. a tarp over the top from the bullbar or a tree fixes that though.