I have just had second thoughts and selected ALL the photos, clicking cmd-opt-I and believe it or not, just one Get Info window opened representing all of them!
One deletion and every thumbnail was renewed.
WARNING TO OTHERS! Do not control-click all the pictures and select Get Info otherwise your desktop will be inundated with hundreds of Get Info windows.
Luckily I was aware of this and did not do it!
If in doubt always check with a few photos selected to see what happens.