In reply to Stone Muppet:
I would go for the Thermarest AirTap Pump (as in just the £10 jobby you punch through any old air-tight bag). I used it on the Appalachian Trail and the plastic bag it came with only broke after about 4 months of using it every night. When that happens, a garden refuse sack will do the job.
I have the big orange pump sack also but it's quite fiddly to get over the valve and I'm convinced it will break soon at that point.
The Airtap on the other hand makes the part likely to break (the bag) completely replaceable at pennies.
It's also a fair bit lighter and a third of the price.