The closest I've seen is this:
http://www.ebay.co.uk/itm/Cycling-B..._Cycling_Bags_Panniers_SR&hash=item3f0eda9007
Which has 6L space, is removeable and is about £20 from
eBay but is Chinese
As I said earlier I ordered this one on the 1st of January and its arrived today, to be fair it has been Chinese New year/Spring festival so will not deducted too many points for waiting a month and a week.
It appears quite sturdy and states it has a weight limit of 10kg, the pannier sits in a metal frame and is held in place with the Velcro pockets
Held on to the seat post with a quick release
Nice size for me 6litres, enough for my bike tools, spare tubes, a packed lunch or spare clothes if going to work.
Zips aren't the best and can see these failing in the not too distant future.
Quite happy with it bought at £16.99 free P&P, not sure I'd pay the £25.99 free P&P
Cheers Sean