The buttons are connected to a PCF8574 i/o expander on the I2C bus. The PCF has a dedicated interrupt pin which goes low if there is a button press. This pin is connected to a defined interrupt pin on the Due. I presumed the PCf layer would provide some debouncing. It worked smoothly on the Mega. I have no scope, but I have a logic analyzer on the way in the mail. I will also experiment with some hardware debouncing for the line.