Most likely the 16K "rampack". It was pretty much essential to get anything done with the machine[1]. The rampack infamously suffered from "rampack wobble" where a slight movement of the machine would disconnect lines between the motherboard and the rampack causing a crash. Eventually I got a larger case/keyboard and mounted the rampack internally.

[1] Though there was 1K chess, a mostly complete chess game in 672 bytes https://en.wikipedia.org/wiki/1K_ZX_Chess

That was it! And I remember the wobble well. Had it set up on the kitchen table and the slightest knock would screw everything up. Good times.

Did teach you to save frequently though, on cassette tape, which only took a few minutes! :-)

Cassette tape? We didn’t have such luxuries until we got an Acorn Electron a few years later! So jealous. :-)