Thank you for all your comments.
I am not taking legal action, just seeking legal advice and am learning from the entire process. I think it is always good to be aware of your rights as it gives you confidence in arguing your case with the establishment. As PandasRock said, there is actually a law broken given the circumstances. The lawyer I spoke to said he will research further on certain details of that law. I am subscribed to Which? Legal which costs £43 for a year for an unlimited , and there is the Retail Ombudsman that can be approached at no cost to the consumer (but with a cost to the retailer). I felt I have gotten good advice already from one phone conversation with Which.
I would not be making a big fuss this time if not for the fact that it is not the first time I have encountered such poor service from them. A few months ago on the shop floor, the attendants were simply dismissive (while gossiping with each other), sending me ping-ponging between tills, each person at each till saying they were not the desk to check out with at the time. It was only when I finally glared at the floor attendant that she realized she had a job to do. BTW, I was purchasing discontinued chinaware (plates) I chanced upon on sale there and had them delivered. Needless to say, that delivery was an entire botched job as well! When I called customer services, they had no clue where half of the undelivered items were...I had to be the one to suggest to the person on the phone that there might be some tracking number (different from the tracking number given to customers perhaps?) they use between them and their courier...and so there was. All eventually got sorted, and I received a letter of apology from F&M promising of course that it will not be repeated etc, etc.
Then this new situation. It seems so straightforward purchasing at the store--my son and I and one of their attendants physically counting the items, taking them to the till and paying for them, including the delivery charge. Btw, again by chance, items from the same range of china I had bought earlier were on offer so I took that opportunity to purchase from this discontinued range (thus not replaceable when gone unless available on ebay and such...and I've looked...none so far). How could anything go wrong AGAIN? It was not even a thought! So a portion of MY goods/MY property were technically disposed of/allocated to someone else and I am the one left with a now incomplete lot.
It doesn't end there. Naturally I tried to arrive at a resolution with them, assuming that after the first items from my lot went missing, that they would protect the rest of it and keep them intact, especially as they had vowed a complete investigation was being made as to how all happened. Well, as I agreed to a resolution to purchase the rest, it turns out, the remaining lot was further cannibalized to a number that makes it simply impractical to bother with.
I don't know whether "smug" is the word but there certainly is a high degree of complacency on their end, and in my particular case, I can describe as a wanton disregard of my rights as a customer. That's why I wondered whether this was just my (bad) luck with them or whether others have experienced similar.