The parking described in the guide (same as for Warland quarry I believe) has been shrunk by new fences. The farmer uses it as a turning spot and isn't keen on people parking there. I had a chat to him last week and while he was very friendly, I decided to park at Cow's Mouth as it just causes less hassle all round.
Cow's Mouth parking adds a bit of time, but is flat all the way. Just keep walking past Cow's Mouth and the reservoir soon comes into view.
--just heard from someone else that the farmer put a polite note on his car asking him to park considerately as he couldn't get out to feed his calves.