Seems I have worked this out. We are newbies on Mongo so unfamiliar with the roles needed.
I removed the root@admin role and we got lots of errors such as "not authorised for querying" etc in the tyk-dashboard log.
Looking at the Mongo docs I had granted the dbadmin role however that is only for administration activities.
I have now changed this to dbowner and it appears to be working.
It would be useful to have a recommendation for what role is actually needed and for this to be in the documentation somewhere rather then us having to guess.