After fixing a minor wiring mishap (I managed to wire up the ALU to bus in the wrong order!) the ALU worked first time without problems. It looks messy as heck, but it is a messy component...

I've uploaded a short demo of the ALU in action - adding B to A then loading the result back into A each clock tick, then I change the jumper for subtraction and count down. It feels good to see some blinky lights happening at last! 8^)