Steve:

Figure 3 illustrates a resistor network, which changes a
4x4 keypad into a switchable resistor matrix. The
resulting resistance is read by an ADC or a capacitor
charge timer, which determines the key that is pressed.
This is practical only when one key is pressed at a time.

... so that doesn't really work in this case, but those are good links from Microchip, thanks

Al: I think the usual solution is to use diodes like you say.

Tim: I'll have to think about that

Thanks for the inputs

Andrew