The Solution!
After much deep research, I finally found a post that led me to change the settings for the Desktop picture.. I had mine set to change every 15 mins, which I think triggers the Finder window to open at start-up, as it has to look for the next replacement picture. So just do System Preferences>Desktop and Screen Saver> and uncheck 'change picture'. Incidentally the Search facility for the forums seems a bit eccentric: it kept throwing up unrelated items. I had to scan lots of pages by eye before I found the solution. This took literally hours.