After publishing the post, I had a look to see if I had dreamt the scenario up.
I couldn't find anything about Nerds and glass from the UK, but I did find a story that was on the Los Angeles Times website. It was published in November of 1988 and documented that the sweets had been recalled after glass was found in a box given to somebody as a trick or treat gift in Pittsburgh, Pennsylvania.
The article closes with the words 'it was not known if the 9-once box of candy was tampered with, said police'.
I know I have many years of experience from that 1988 story, but I'd guess that it was indeed tampered with!
As for the UK - I wonder if that exact story made its way to our shores and that's why there was a panic about Nerds and glass.