Seems the genuine D1 mini pro is in short supply, so I had to buy a clone!
The problem is the clone uses CH340 for the USB to Serial whereas the original uses the CP2104. It also has no series resistor in the Rx path.
CH340 when the USB is not connected drives the RX pin low. This blocks all serial traffic from the hottub.

The solution I came up with was to use SoftwareSerial on pin 4 & 5, connected to the RS485 converted.
Running Software Serial at 115200 baud is at its limit, but it works with only a few errors every minute. These are normally quickly overwritten by correct data.
Maybe I'll add better data checking so these errors don't get passed to MQTT.

Has anyone else had problems with clones?
Any other the solutions than SoftwareSerial?

Ray