Cortech makes good stuff that's high value for the cost. It's not super fancy, but good value. They aren't waterproof like Kriega stuff, but do come with water proof slip covers. They offer a number of different size tank bags with straps (as opposed to magnets)
- If she's not very tall, a 10L bag might be a bit big - of course that will be tough to figure without a 'test drive'
The cortech back shown by IZ is small, but fits a remarkable amount of stuff. And has a clear panel on the top for maps/turn sheets.
Kriega makes some very nice stuff. It's more Adventure type stuff - but very good. As mentioned it's robust and water resistant. They do make some water proof items - but not all of their line is. Even the items that aren't water proof are very water resistant when all closed up.
As noted, Kriega offers modular set ups so you can tailor the volume.
Kriega doesn't offer a specific tank bag though. They make a mount that goes over the tank that you can attach one of the same packs used for the modular tail pack. back pack add on.
I know this isn't an EU vendor but they stock a lot of bags and might serve as inspiration/research
http://www.twistedthrottle.com/- IF you spy something good in there, you might be able to find an EU vendor that stocks it.