MacAllcock: Yes I have the manual and note that although there is a Centre IN and a Centre OUT, nowhere could I find information as to whether the "normal" signal is being wired to the Centre OUT (that is, without a dedicated Centre IN signal). Given that it doesn't work I guess that proves it!
Kingfrog: Now that is a suggestion I will have to think it through. The mixer does have a Aux OUT.

I have another work round by taking the K1800FX straight from the mixer, rather than via the sub-woofer. A circuit diagram would be helpful if I could find one.
Thanks
Keith