Yeah I did.
After my last reply I ran apt-get install mono-devel again to see if it would kick in those packages being held back, it installed stuff from both repo's. I then was able to run apt-get upgrade without it giving me dependency errors.
Perhaps there should have been and apt-get purge in there somewhere in relation to clearing out the old mono-devel before replacing it?
I've restarted the service and am now waiting to see if it shows as enabled/active and I get the dashboard entry.
edit - woo, the sonarr service is green and enabled now at least. refreshed the page to get the dashboard menu entry for it.