Thought I was doing something stupid, but no, lots of other people are having the same problem.
I created a big playlist - on my iPhone, realised it was too big removed a bunch of songs, probably about 50.
Later realised these had been deleted from the iPhone.
When viewing the music on my device through iTunes on my PC, the songs still show as being present, but when you click on one, it can't be played, because it is no longer on the iPhone, and a little exclamation mark in a circle comes up alongside the song. I then have to delete the song from the iPhone (even though it already isn't there), then copy it back from the PC to the iPhone.
My songs are not purchases, I am old school, I buy CDs, then add them to iTunes and copy them to the iPhone. It is taking ages to go back through my playlist - by memory - to work out what I added and then removed.
Thanks Apple for your total balls-up. Please change this functionality back.
If anyone knows a way to identify the missing songs, it would really help.