Eurohike Packable Daysack - £3.99 @ Millets
Can get it online too - use fLM4yK for £1 off online - free delivery (confirmed by Loudmouth)
When you're travelling, you don't want to lug your rucksack or suitcase around all the time: it's far better to leave it at the hotel, hostel or campsite and take a daysack with all you need for the day. This lightweight packable travel daysack folds down into its own stuff sack for easy packing in your luggage. It has a 20 litre main compartment with a dual zip.