Another good company for pannier bags is www.maneevent.org.uk
& are based in Manchester. They mainly produce inner bags for BMW & Hondas, but look as if they can cater for other makes upon request.
I bought a set of inner bags for my GS panniers about 4 or 5 years ago, although not cheap the quality is excellent and at the time they were at least ½ the cost of the BMW equivalents.
They even make expandable ones to fit the variable size panniers and Top boxes that are doing the rounds.