Thanks Hmmerdown, Dee and Ahab!
I tried the pencil thing you recommended - problem was, the barrel is really long and I couldn't find a pencil long enough so that I could see it when I put it down the barrel. So, I folded up a piece of paper and put it between the pin and the cylinder, cocked the hammer, pulled the trigger and looked to see if any impression had been made on the paper. Nothing there.
Next I tried what Hammerdown said and, sure enough, the cocked hammer drops when you push on the back of it.
Not good.
So, I've locked up the pistol and will not be using it until it is repaired.
Thanks very much for all your help. Have a Merry CHRISTmas, and remember the birth of our Saviour as you go about Christmas busy-ness!