OK I have fixed it, but if anyone else is interested the problem was caused by the user selecting and copying the word he wanted to replace from the formula bar, then he would click off the cell to get out of edit mode. Excel will take this as an edit to this cell even though nothing has been changed, and since all the other worksheets are selected the cell is pasted across them!! The Find/replace part was all a bit of a red herring!
Cheers