A quick bit of boolean logic algebra proves that a single 74C00N Quad NAND gate chip will allow me to make one NAND gate and one AND gate, and hence generate two separate opposing logic HIGH signals from the single LOW/HIGH output of the Mk2/4 board. Have ordered the required chips and header boards from eBay, which can also be used to the two opto-isolator boards for the proximity switches.

Adrian.