Welcome to the forum mezz,
Here are a few contact details you could try -
For Morton's Leam, Twenty Foot River, The River Nene east of March, Sixteen Foot Drain from Boots Bridge to Three Holes.
Try March Working Men's Club, Tel: 01354 653223
Also you could give Peterborough & District Angling Association a ring Tel: 01733 571676 and try to poach some info from them
Regards, Dave