Thanks, Pecoflyer. I have looked at the link.
Although the explanation is clear the fixes are either inaccurate or inelegant. But here is a curiosity: if you take an empty cell and enter =17.73-25.00 you get -7.27, which is correct.
So the link explanation does not really solve the problem of why in my spreadsheet G18-F19 gives the wrong result in G19. In neither G18 or F19 are there any floating point errors, and Excel has shown it is perfectly capable of doing the calculation correctly!
Richard