Nothing has changed at my end, apart from automatic updates to Windows and Office/Access. I originally chose .xls because Excel 2000 was the latest version mentioned in the Access help file. However, since adding that import I have added elsewhere in the same database import of a different spreadsheet of .xlsx format using the same DoCmd.TransferSpreadsheet method with no problems (the only differences being the spreadsheet imported and the table into which it is imported).
The Access visual basic code line I am using is:DoCmd.TransferSpreadsheet acImport, 8, "tblAddressesImp", "C:\SpreadsheetFolder\Valid.xls", True
As mentioned earlier, it initially worked, but since January this year has not.