Thanks for all your comments, my folks did say that they thought the whole thing was dealt with fantastically well, and that they were well-informed. However, it has shaken them up a bit. They were in Club (a one-off treat, not their usual situ) and they did think it was turbulence at first, but at the end of the day the passengers were the lucky ones.
I think it's important to remember (certainly from a Pax pov) that the fact the flight arrived safely and comfortably at its destination is really a credit to all on board and their professionalism. It certainly won't prevent them flying BA again as they were impressed by the handling of the situation.
My intention when starting this thread was to get some info on what happened to cause the event, thanks for those insights!
Get well crew!
Pax72