Thanks again, folks. Food for thought.
The vast majority of my numeric inputs get passed to a long-established subroutine that works like a charm, although I've recently had to tweak it to accept zero values as genuine.
Pure alphameric input in my programs is unusual, and in the original 1980s code, things worked. (For reasons made clear here.)
There have been some useful suggestion in this thread, and with a couple of further programs on the stocks, I will be trying them out. Call it the Covid-19 dividend: after many years of almost zero programming, there's something of a personal coding flurry underway at present!
Malcolm