Finally fixed it. I had to go in to mongo and issue the following commands
use rocketchat
db.migrations.find() (not totally necessary but checked for duplicate records)
db.migrations.update({"_id": "control"}, {"version": 231, "locked": false})
This allowed me to roll back the version to 3.18.1. I then went to administration/apps and uninstalled the poll app.
After this I managed to update to new version.
Thank you everyone for your help I hope if anyone else comes across this problem this helps.
Duncan