Or you could put it in the mod_kl_php and publish that module to the first module position that the template loads. That way it you could run the session_start() code on just the pages that you are using it for.
Success!
I decided to first try the code in a new test site and it works like a charm. I'm sure with a bit more time I'll also get it working using Donna's latest suggestion.
Thanks to everyone - especially Donna, who's help has been invaluable.
True to my word I'm willing to pay for this assistance, Donna. Or, if you'd prefer I'll make a donation to a charity of your choosing.
Again, many thanks.