Carradice Saddlebags

I am thinking of a super c saddle bad for everyday commuting. 23 litre capacity sounds like enough for most stuff I'll carry with room to pick up groceries on the way home.

What are people's opinions on mounting options? I was thinking about the SQR system but I don't know if it will work with such a big bag. Also are there shops in London that stock them as I'd like to see one before ordering

A saddle bag that big and full is horrible, it sways and really influences the bike's handling. I carry one of those tiny folding rucksacks in the saddle bag and when I have to pick up the shopping use it for the stuff I can't get in the saddlebag
The SQR is great if you don't mind the bracket left on the seat post and the slightly naff way they attach the saddlebag to the other bit. It works well enough, but it isn't cutting edge carrying technology.
I use a Nelson and a bagman Q/R saddle support, and I can get a lot of stuff in it, more than I need for a day ride.
I reckon a saddle bag greater than about 12 litres is pointless. Those big bags sway like a drunk when full and flop about when part full.