Umm , oops , maybe we led you in the wrong direction , but then the battery was old , but if you had of mentioned that the terminals were all corroded it may have been different , having said that , whoever fitted the secondary pump /controller must have seen that and chose to ignore it .
Clean up all connections and give that a whirl , look on the bright side , you can now run each system with its own battery , possibly even feed them both off the SCR , ok its cost £99 but its not yet wasted money is it ?
And it might be worth tracking the wire back to alternator and cleaning them connections too .