Sorted!
OK I was told about Putty ssh and applied the fix. All is well again.
I am puzzled why this old problem came back again after all this time when plesk updated itself at the weekend! Can they not fix it at source and send us updates that do not clobber us each time!
Sorry I am new to all of this. I have what I think is this problem and want to try this fix. So where do I go to enter these commands?
Also this happened once before and was fixed but is now back again. Will it happen ever time our server is auto updated?