I do not need to do any calculations in the yearly sheet. This is just for a collection of existing data. All the calculations have been carried out in the monthly sheets.
In the yearly sheet I enter the path, the file name and the cell reference for a closed monthly sheet. Upon pressing 'enter', a screen pops up telling me to open the relevant sheet by clicking on the file name and then click 'OK'. This has to be done for every data cell in the yearly sheet.
If I the open the sheet to display on the screen, the path does not disappear. It remains as part of the formula in the yearly sheet.

However, If I start afresh with an already opened month sheet and just manually enter the file name and cell reference (no path) in the yearly sheet, it works fine. If I close the month sheet, the path appears. If I open the month sheet the path disappears. Which is as it should be.

I've also found that if I try a 'Find and Replace' to change all the formulae to a different month, say, AUG to JAN, the the pop-up sheet appears for each cell change and the full path is added even if the month sheet is open. But if I manually alter each cell, then it is OK.