If this is the path that runs along almost the edge of the reservoir it has been 'closed' for some while now
There was a sign at the car park end at one time. I think it was where the permissive path led off the bridleway from the car park.
I'm pretty sure I have walked it since it was closed by going a little further south after crossing the bridge then round the corner of the field boundary. My map shows two points of access but as the fields are not within the open access moorland and there are usually ponies in them I can understand the path being closed.
Did you go through Black-a-tor copse? I think it's a lovely place.