Bevan:

I had the problem of split phases as well. I found that if I plugged the IR543 into a different outlet, I could get it to work (on the other phase). Luckily, the other outlet was also accessible from behind my equipment cabinet. Try moving the IR543 to a few different outlets in your house. Sooner or later, if it is a phasing issue, you will find an outlet that will work. Even if it isn't practical to leave the IR543 in this location, you will have isolated that the problem is related to phasing. Then you will need to find an "X10 signal bridge" to solve your problem.