Thanks again @blh64. That has solved that error. I have now got the sketch compiling.
I have now found that when I set the correct time etc and then unplug the usb cable and then re plug it in, that although the backup battery in the RTC is operational, the time display is turned back by 1 hour. Each unplug and replug of the usb sets the time further back by 1 hour.
I presume this could be the code recognising overall power failure, despite the presence of the battery in the RTC.
I will need to give this a bit more thought when next working on the sketch.
As usual any suggestions would be most appreciated.