I reworked the board to allow for a wide ZIF socket as there are wide "universal" Zif sockets that allow 0.3 and 0.6 chips giving more versatility.

Also corrected a hardware error where I had mistakenly taken INTA and INTB on the MCP23017 as inputs rather than outputs. The previous hardware tied these to Vcc using a 10k resistor. The earlier revision of the board seems to work in practice despite this, however now I've left these INT outputs as unconnected.

Also, I didn't like the way that the layout software was working so I made a minor mod. My PCB has wide power tracks and in some cases I have MCP23017 contol lines tied to Vcc or Gnd and this meant the fat trace came right up to those pins which cluttered the layout and caused clearance problems. So I included a 0ohm resistor in front of those pins to terminate the layout rule and drop back to a narrow trace. In the finished product this will be a jumper. Perhaps I could have done the same by using pad to separate the two traces? Anyway, now I have no problems with clearances.