Looking at my copy of the schematic - U21 is a MC14584BCP.

Uses pin 13 input from the crystal, 12 out to pin 70 of the custom chip (This should be the 32khz clock signal? Marked 32khz on the diagram), pin 14 Vcc which measures 4.48v with the board removed and not powered on. I have checked continuity between the parts of the circuit - all checks okay - no corrosion up there? Bearing in mind I am an an idiot.

This thing has has corrosion repaired in the past (looks like a good job too) - U18,19&20 all socketed and the remote battery mod. Wrong end of the board though?

The only thing with the RTC circuit was the xtal which had come adrift? (It now has continuity to both sides of the board)