Andy, I had seen this in the Echo but was not intending to visit until I read your post. This morning I parked at the Andoversford end (room for 4-5 cars and took the path towards the reservoir which I knew went along the back of the reservoir. The first part was surprisingly muddy, but I used my monopod as a walking stick and got through. There are only a few points that you can access the edge of the reservoir and it is well shielded by trees and shrubs for most the length. This side would be best for early morning and late evening (at least in the Summer). I decided not to retrace my steps and walked back to my car via the A40. It was easier to photograph over the fence from about halfway along and I also saw a couple of Grey Herons and a Cormorant. The sun was then behind me which was better. I did take old shoes but wished I had put my boots on. With the rain forecast over the next few days, it will get even muddier.
Thanks for the tip.