Just to wrap up.
iRule architecture assigns one gateway (basically an IP address) to each device (set of commands for a piece of equipment). Each gateway will only support one mode of communication - basically IP or IR. My requirement meant using both IP and IR to communicate to one device (my receiver). I therefore had to set up a second device to handle the IR command (iRule now thinks my receiver is two devices) and assign an IR gateway to it. It would seem that this is not often done in the world of iRule.
Having confidence that I had the correct IR code helped a lot in uncovering the underlying issue.
More detail in my post "Onkyo Power On" in the iRule forum.
This question is closed. Thanks again guys.