Check out the Ventura stuff made here in NZ. This is their 20ltr seat bag.

http://www.ventura-bike.com/products.aspx?code=SB0622

They also make a very tidy bag system which i've used for years on many bikes.

If you can't get it (at least for a reasonable price) in the UK, I don't mind sending it over. The Imola bag costs about 40 quid here.

Steve