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It's this one: http://www.roswheel.com/saddle-bag/bicycle-saddle-bag-attack.html
The seatpost end of the bag is kinda semi-rigid and quite narrow, the rest is cloth, with a waterproof lining. The inside's kinda sticky, for want of a better word. So it takes a bit of effort to shove stuff down into the bag. It's one big bag, no compartments, allegedly 10L. There's a little stringy bungee thing on top, so I guess you could strap a Ti cup to it ;)
Got it from Aliexpress - I can dig out the seller details if I find a minute.
My aliexpress Roswheel waterproof bag turned up.
It feels alright, pretty well made. This is it packed with all the stuff I need for commuting.
Only maybe dodgy bit is the backpack clicky things don't look particularly sturdy. I'll give it a try tomorrow.