I'm looking for a Travelpack system rucksack/backpack with a detachable day bag, locking zip fasteners and covered straps. The size I'm after is somewhere in the region of 60 - 80 litres plus 15 - 20 Litres for the daysack.

A normal rucksack is no good as it is top opening and doesn't normally have locking zips

I've seen various offerings from Berghaus and Karrimor for between £90 and £150. Does anyone have any good recommendations on which to buy, and more importantly where to buy it from?

It needs to be sturdy enough to wthstand being kicked around by airport baggage handlers, and thrown around on trains and buses on my travels.

Thanks