I've now built a second keyboard unit for my big RC2014 system and added Fuzix support to it. This works nicely and confirms the fact that the way the membrane is designed means you don't need to debounce it. I can now run the RC2014 system with TMS9918A video and the keyboard turning it from a serial based machine into a proper 8bit home computer.
One building lesson learned however. If you can put the connectors both ways around always build it both ways before concluding you didn't miss anything.
In this case the 5pin ZX81 connector is very close to the resistor pack with the connectors facing down, but fortunately if you fold the resistor pack over a bit then it all fits.