@Lucius and @TryNinja
Many thanks both and that seems to have done the job - yay!
I'm going to put in a good word for the University of Cyprus who have been running a series of MOOCs in Cryptocurrency. I'm trying to work my way through one at the moment. I guess I'm learning a lot, but also learning how much I don't know...
...hence my cautious question to this forum as it's so easy to make a major, ahem, blunder!
Thanks again for your help.