Thanks, but I think the angels are with me. I came upon a post discussing a similar (but not exact) problem somebody was having.
They developed the idea that forcing a write of some data to the suspect drive, the issue could be resolved.
The logic of this eludes me, but such a simple action was worth a try.
And voila, the parity was clear and my concerns gone. (I am however not too keen on deleting that data I wrote - just in case).
I appreciate your help and suggestions which I will bear in mind for the future.
Baregills's post in 5 parity errors that will not go away. was marked as the answer