Well the developer did reply to me and offer help - which was much appreciated
However, as it turned out, I didn't need to actually try
any of the suggestions because in the meantime I had decided to re-download the game files - and this second download worked perfectly !!
What went wrong with the original downloas is a mystery but I do now have a working game
I've only just been able to get back to my computer to report - but I have now emailed my thank you to the developer and also, of course, to you, Chief