So this week I've soldered the low-voltage side of the SparkFun converter board 'onto' the Raspberry Pi. I've let four long leads run freely from the high-voltage side of the board that can be pushed into the pins of the Psion top slot (breadboard-style!)

I've put the connections so far into a table. Now I need to put a fresh Raspbian Lite on the Pi and try to work out how to 'use' the Psion top slot. I believe it has 3 general purpose pins that can be read and written to using some kind of machine code. This page is proving very interesting https://www.jaapsch.net/psion/topslot.htm#p16.