I have carried on and using the search process.env.NODE_TLS_REJECT_UNAUTHORIZED = from @omrid answer, I found this thread
I imported this into my flow and modified my HTTP request to the same as the 'Self Signed' HTTP Request node in this flow. It didn't work.
Copied the 'Self Signed' HTTP Request and inserted in place of my node and it worked. I have now studied both my original node with that from the other thread and can find no differences. I have no idea why it wouldn't work.
Funnily enough, one of my 'fumblings' yesterday was to create a TLS check as @knolleary had indicated (without seeing his post). So I am please I was on the right track.
Thank you @omrid and @UnborN for your pointers! I couldn't have done it without your help!!
