Neil,
I'm up in King's Lynn, so fish most of the drains, Ouse, Roswell, Wissey Pools etc.
I've seen them at both ends of the Delph during summer, the Ouse and Roswell Pits - a chain of lakes attached to the Ouse near Ely.
There were some seriously good ones caught on The Lark this season, best was just over 30.
I also know the Nene's got them in around Peterborough, but I've only fished this a couple of times in the winter.
It's probably more of a pipe dream than a serious goer but if anyone wants to know where I saw fish I'm happy to pass it on.