Received my Southampton tug from Howes today

I'm very pleased with it and works straight out of the box with the supplied radio, however.....
As I have spare 2.4ghz receivers and want all my boats running on 2.4, I unplugged the 27mhz 2 channel receiver and changed it for the 2.4.
Somewhere I'm going wrong. I can get the rudder servo working off the correct stick, but try as I might, the motors also want to work off the same stick, even plugging the wire into channel 3, or 4 or 5 or 6 has no effect?????
Oh yes, I AM using my 2.4 transmitter

Any ideas chaps?
Ta so
Nige