Topeak Wedge Clip on medium - cheapest UK price I can see and cheaper than smaller ones!
Replacement brackets for this system seem to be around £7-9 alone.
Loads of stock in all my local stores £11.99 at
Halfords. Comes with a yellow raincover which is perhaps superfluous, and it's "expandable" via zip.
http://www.halfords.com/webapp/wcs/..._productId_220315_langId_-1_categoryId_165643
Tried fitting one of the German Supermarket specials the other day that I have picked up over the years - looked an absolute mess - but does the job fine and it came with a useful multi tool and levers - rest of the stuff was bin worthy.
I'm already breaking The Rules by having one of these but this looked like a good alternative.
Fitted a tube, levers, vinyl gloves, multi tool, rain cover, emergency tenner in one of those heavy and expensive "stash tubes" - I know it'd go flying if it wasn't, an energy gel, tiny repair kit of patches and a bit of sandpaper and chalk and my massive set of keys with 13 items on em and a 1/2 Watt Smart light in the clip tab (tight fit)
Gonna have to reassess that number!
Nearly 800 grams all in so not ideal but easy to remove and a lot more comfortable than a backpack or jerseying all that crap.
Not too sure I'd be using this for off road, even though on road feels like it sometimes is. Loads of vibrations and jumps etc are going to wear things like this a lot quicker. I'd just wear a backpack for off road but maybe that's just me....
HTH - happy riding!