Thanks Gil, at the expense of appearing "dumb", I found that on the xLights layout page, with the polyline model as a contiguous chain all was well. If I attempted to place a new universe/start channel (Universe 44:1) to the point I wanted to break into the chain, to match the setup configuration, I would have had to apply the same, incremented, for each item on the list from that point on in the chain.
I re-arranged the setup page to address the start, and the middle section of the Polyline model, where I would like to inject the data from another output of the Falcon, and xLights seems happy :-)
I have been pondering the "problem" and if I am correct, I should, as suggested by yourself a year ago, be able to simply apply the extra Falcon output part way along the polyline chain without changing anything so long as the numbers are correct, before and after that point.
I suspect I may have been over thinking the whole thing :-( I would like to apply the PC/xLights to the Falcon, but at this stage it is still packed up in it's box as we are well away from home, and I simply doing planning for the display attempting to get my head on straight!!!
I have pasted below a snippit of the Layout page, where the chain involved is located, as well as the setup page to illustrate the configuration.
Thanks for your time, I am still getting my head around this gear :-(