Thanks one and all, I'm very grateful to you.
Having had the memory nudged in the right direction, I recall the red cross on the tail viewed through my dad's recently acquired, and somewhat basic, binoculars.
I've found a picture of a Convair C-131A Samaritan at Northolt in 1971. I'm not going to post it here as it is copyrighted but here's the link:
Air Britain.
Thanks, chaps
