Thanks for the suggestions. I like the look of the Lone Peak bag but they seem to a bit of rarity in the UK. I popped into Evans Cycles yesterday with the mezzo and looked at Topeak and Altura bags. The Topeak is out of the question as it looks like it only fits Topeak carriers. The Altura looked promising and the side pockets convert in to small panniers, I think this bag would clear the pedals.
I have also come across this Mezzo review http://www.velovisionmag.co.uk/showStory.php?storynum=1240 The chap was using small panniers on the Mezzo rack, albeit on the old design of rack.
Another possibility is to use a quick release seat post rack on the mezzo, a little bit more fiddly and they usually have a limit of 10kg.