ok, we have just finished madoc's campaign in france and my knights decided to stay with praetor. anyone had this happen?

Hope they all hired stewards to oversee their manors, else they won't have much to which they can come home. And they'll need ample cash to hire passage home after King Claudas sends Praetor Syagrius and his allies packing.

My knights didn't even bother going to France, choosing instead to stay on the mainland and heading off to Somerset.

Still, with a bit of work you could expand that to include some quite tasty battles and sieges, eg in and around Rouen.

I was thinking about expanding it a little bit. Battle or two and of course Clovis kicks Syagrius' ass but maybe I would love to give PCs a skirmish or two to have a taste of some victory and not only crushinng defeat. Are there Frankish army lists in BoB?

Book of Armies has got details on the Frankish armies.