/ Waterproof kit bags
These are a bit more expensive but utterly indestructable..
Yup very good, obviously not a replacement for a North Face duffel or whatnot as they only close by a draw cord, but very handy for chucking in the car
If you want something more watertight try these http://www.ewetsuits.com/acatalog/rucksack-dry-bags-rucksacks.html
Taken it on a couple of expeditions where carrying for a longish distance was required and they did the job nicely
Thanks for the advice guys, I joined their mailing list last week and have just received a voucher code for 10% off, makes it a no brainer, just ordered two. Voucher code is DUFF10 valid until end Nov