I am unsure about SSSI except (Google) it makes interesting reading in respect of the lake and I suspect bulldozing trees into it would have been well frowned on & the "landscaping" is questionable where it has infilled the lake along one side.
I wrote to NRW but got no reply...........
I suggest you park at the church, as it's not a great road. Approach from Lampeter direction for turn into church. Footpath goes across field, and straight down to lakeside, but no further.
As to objectionable landowner - that's a difficult one, but I had a full lecture on shooting, fishing and falling in, etc! It has put me off but it is a good site.