Thanks to all who have given their advice on the upcoming visit from the Jehovah's Witnesses. Just from taking their pamphlets for all these months, I think if I were going to be dismissive I should have started that from the beginning, so I'm going to see the conversation through even if it does turn out to be a clunker. And as you all have said, any argument which is over their heads is one they're likely to take back to their supervisor rather than ponder themselves.
So I'm thinking I may start with what they do acknowledge. It seems that their usual script covers retranslating "the Word was God" into "the Word was a god" and expecting the Christian (Nicene) to drop it there, rather than run with it. If they translate the beginning of John as "the Word was a god", I think I could ask them exactly how many gods there are, and whether it's ok to worship this god. I'll also see if they have any understanding at all of what the Word of God becoming flesh really means and why it's big news for who Christ is. If they bite, I might even explain how Christ replaced the Temple ...
Comments and advice are welcome. So are prayers.
Thank you much for the comments and advice. It's been helpful in putting together my thoughts.