Thanks again for the advice. I added
persistence_file: '/config/mysensors.json'
to my configuration.yaml file after deleting mysensors.pickle and things seemed to work much better as I didn't get the initial error. However I did get these errors instead:
2020-03-24 19:45:34 INFO (MainThread) [homeassistant.setup] Setting up mysensors
2020-03-24 19:45:34 WARNING (SyncWorker_3) [mysensors.persistence] File does not exist or is not readable: /config/mysensors.json
2020-03-24 19:45:34 WARNING (SyncWorker_3) [mysensors.persistence] Trying backup file: /config/mysensors.json.bak
2020-03-24 19:45:34 WARNING (SyncWorker_3) [mysensors.persistence] File does not exist or is not readable: /config/mysensors.json.bak
2020-03-24 19:45:34 WARNING (SyncWorker_3) [mysensors.persistence] Failed to load sensors from file: /config/mysensors.json
2020-03-24 19:45:34 DEBUG (SyncWorker_10) [mysensors.persistence] Saving sensors to persistence file /config/mysensors.json
2020-03-24 19:45:44 DEBUG (SyncWorker_2) [mysensors.persistence] Saving sensors to persistence file /config/mysensors.json
And also I still can't get past the original point in the communications but perhaps that is now linked to the file permissions above:
2020-03-24 19:45:34 INFO (MainThread) [mysensors] Connected to Serial<id=0xb272eab0, open=True>(port='/dev/ttyUSB0', baudrate=115200, bytesize=8, parity='N', stopbits=1, timeout=0, xonxoff=False, rtscts=False, dsrdtr=False)
2020-03-24 19:45:35 DEBUG (MainThread) [mysensors] Receiving 0;255;3;0;9;0 MCO:BGN:INIT GW,CP=RNNGA---,FQ=16,REL=255,VER=2.3.2
2020-03-24 19:45:35 DEBUG (MainThread) [mysensors.handler] n:0 c:255 t:3 s:9 p:0 MCO:BGN:INIT GW,CP=RNNGA---,FQ=16,REL=255,VER=2.3.2
2020-03-24 19:45:35 DEBUG (MainThread) [mysensors.handler] n:0 c:255 t:3 s:9 p:5 TSM:INIT
2020-03-24 19:45:35 DEBUG (MainThread) [mysensors.handler] n:0 c:255 t:3 s:9 p:7 TSF:WUR:MS=0
2020-03-24 19:45:35 DEBUG (MainThread) [mysensors.handler] n:0 c:255 t:3 s:9 p:15 TSM:INIT:TSP OK
2020-03-24 19:45:35 DEBUG (MainThread) [mysensors.handler] n:0 c:255 t:3 s:9 p:18 TSM:INIT:GW MODE
2020-03-24 19:45:35 DEBUG (MainThread) [mysensors.handler] n:0 c:255 t:3 s:9 p:21 TSM:READY:ID=0,PAR=0,DIS=0
2020-03-24 19:45:35 DEBUG (MainThread) [mysensors.handler] n:0 c:255 t:3 s:9 p:24 MCO:REG:NOT NEEDED
2020-03-24 19:45:35 INFO (MainThread) [mysensors.handler] n:0 c:255 t:3 s:14 p:Gateway startup complete.
2020-03-24 19:45:35 DEBUG (MainThread) [homeassistant.components.mysensors.gateway] Node update: node 0 child 255
2020-03-24 19:45:35 DEBUG (MainThread) [homeassistant.components.mysensors.gateway] Node update: node 0 child 255
2020-03-24 19:45:35 DEBUG (MainThread) [mysensors.handler] n:0 c:255 t:3 s:9 p:28 MCO:BGN:INIT OK,TSP=1
2020-03-24 19:45:35 DEBUG (MainThread) [mysensors.handler] n:0 c:255 t:3 s:9 p:36 TSM:READY:NWD REQ
2020-03-24 19:45:35 DEBUG (MainThread) [mysensors.handler] n:0 c:255 t:3 s:9 p:41 ?TSF:MSG:SEND,0-0-255-255,s=255,c=3,t=20,pt=0,l=0,sg=0,ft=0,st=OK:
2020-03-24 19:45:35 DEBUG (MainThread) [mysensors.handler] n:0 c:255 t:3 s:9 p:52 ?TSF:MSG:SEND,0-0-255-255,s=255,c=3,t=20,pt=1,l=1,sg=0,ft=0,st=OK:0