Quick follow up
I plumped for el cheapo on eBay:
http://www.ebay.co.uk/itm/272308769491
£7 posted. Arrived today (4days)
Seems reasonably well manufactured. No rough edges, air tight. Materials seem substantial enough for the job for now.
However the 3L is looooong and thin.
Listing images suggest that the smaller sizes are same width but shorter. Not many people going to be using a 3L on a bike, but this is for walking for me. Fortunately it fits in my hiking rucksack which is longer than most ride bags so that’s OK.
Bite valve is interesting design. Valve can be opened or closed with your mouth by pulling or pushing. Once open you bite to allow water to flow.
All in all, it should do the job with little fuss.