Think you'll find it was closed by BR, in 1975. It was not part of the basic rail network, as outlined in the 1966 Network for Development Plans, details on the railways archive website. Another issue is that it was in receipt of a grant under the 1968 Transport Act, for a 3 year period. The local authority took over payment of the grant and in 1975 decided to cease payment of it, the year BR closed the line. Why you may ask; IMHO, it could be that the grant asked by BR to operate the line had increased disporportionally. Council minutes of the time may yield an answer, equally correspondence in the PRO at Kew may also shed light on what happened.