Lowe Alpine - Wadi 10. An excellent 10 lite sack built to body contours for running. Can carry everything necessary for an event, though it's pushing it a bit if you do a high level winter event (needing extra fleeces etc.) Also comes with a bladder, couple of mesh pockets and you can stick waterproofs on the outside with elasticated cord.
By far and away the best sack I have used for running events, think I got it from Pete bland sports. There is also a 15 litre, but I haven't managed to get one yet, think I have a bit of a rucksac fetish looking at my collection of them!!! Matt.

Thanks, Ian. I'm assuming somewhere like Field & Trek or Run And Become will stock them?

the rucksac i use is the Vaude Ultra Hiker 20, i used it on the 100 it's light weight has 2 external mesh pockets an internal pocket which holds a 3ltr platypus, and a zipped pocket on the top the design of the back keeps it well away off the back to ensure good ventilation, the straps can take 16 stone+ running and falling down hills

This has only a tenuous link to events (in that events is where it will get used) but there doesn't seem to be anywhere else on the forum which fills the bill. I want to buy a lightweight rucksack that I can run/jog/trot with - roomy enough to carry water/lightweight fleece/hat/gloves/energy bar etc. Anyone got any recommendations?