Have you tried contacting Historic England and explaining the difficulties you have had with the local conservation office? If not that might be the way forward as well as contacting your local MP about this.
With respect that wasn't your question. Your question was "owner not attending to repairs, what can be done?" A local authority can serve an enforcement notice but you as a member of the public can't make them do that. My answer makes a suggestion as to how you can proceed now.