I've just remembered I never checked the other A/D resistor chain for the
number buttons .

With the other chain , 4.83V V ref , you only have to add a pot and 100K or
so on the line to ground and bring that voltage down to 4.7V and that
disables any key sensing, whether it drops it to 3V or 0V on a key push. So
grime in the general sense could cause that symptom