I asked the same question on another forum a few years ago.
The most helpful reply came from Richard Maund:
"Railway Magazine December 1966 page 720 recorded that the "car ferry" (their term!) service ceased after operation on 6 October 1966 (they do not comment on the curious fact that this date was a Thursday) and that the passenger accommodation was a 1947 Hawksworth slip coach (the last working for such a vehicle - albeit no slipping involved!)."
The following website also contains a lot of relevant information:
http://www.rmweb.co.uk/community/index. ... r-service/