Joshua,
Thanks for the link, looking at the fuzzy image I was able to put in the main() code and run it as debug mode in pyscripter. It's a bit of a fudge it looks like one is unable to run interactively so for example what if your input was a layer with a selection? Fortunately for me I'm not in that scenario so it seems to be working.
But it would be best if ESRI reinstate this important blog page for other people who are looking to debug python toolboxes. It's quite frustrating that when one searches forums, including geonet, that there are references to ESRI blog pages that have simply been lost when they hold such valuable information, much like ArcScripts.
Duncan