Been piddling about with a ESP8266 Node MCU board and a RS232 to TTL adapter, tried a few of the various wifi modem projects floating about, got a few of them to work partially - that is to say communication with commslink, and using terminal to interact with the wifi board, even visiting websites from the Psion (not that it is too useful given the screen size etc.) Sometimes the text is gibberish so there might be some data overflow happening, I think flow control might solve it but none of the projects I tried so far support it as far as I can tell.
I’ll keep experimenting and if I get anything worthwhile working I will post the details in case anyone else wants to try.