The problem is that the woodburner back boiler feeds and returns have
been simply plumbed into the main central S plan heating arrangement
with plain T joints...

Isn't this an application for injector tee's?

http://shop.ebay.co.uk/i.html?_nkw=i...286.m270.l1313

If you want use a swing check valves, BES has a good range:

http://www.bes.co.uk/ (search for "swing check valve")