The basic GCODE interpreter is about 90% coded onto the STM8 part of ESP14, with considerable thanks to @Thomas for his STM8EF forth interpreter. The comms part is also about 90% done on the ESP8266 part, with thanks to zeroflag for his punyforth. So here is a photo of the breadboard of ESP14 being programmed. Still running code in RAM until it works fully.
Now, my job is to get back to the motor driver and get it to communicate with this ESP14. Since ESP14 is a wifi access point, I just need to connect ESP12 as dhcp client and set up the tcp channel through a chosen port (that's relatively easy). And then it is just a matter of getting both ends to behave properly when something (temporarily) interrupts communication (and that is the hard bit).