Well everything is now working! In total I made four (big) mistakes:
1. Used the wrong type of switch for IN.
2. Didn’t use IC sockets (because hubris / impatience / I’m a HERO) and either broke or used a broken 4013 Flipflop.
3. Used a slightly different Static RAM chip, not noticing that an OE signal in the circuit was where A13 was on the chip
4. Used an EEPROM and couldn’t get any data from it - not realizing that again, A13 was always low as it was OE on this board, and so it was reading from part of the EEPROM that I hadn’t put data in, making me think the CPU wasn’t able to read it ‘cos it was all FFs.
Thanks to Josh I was able to confirm the CPU was still alive, and the switch circuitry was working as expected (once I fixed the flipflop which meant unsoldering it and this time using a socket).
Then, once I pulled A13 high on the OE the RAM chip started working. And once I programming the ROM starting at 4000h it started working too!
So now I’m ready to being exploring what this little marvel can do!