Yes, quite correct - Aled Isaf Reservoir !!!
... the water level was very low yesterday and the rock had become exposed. I'd never seen it before and have driven that road quite a few times in the past. I have also rallied along it in the middle of the night, nearly went into the lake many years ago when travelling northwards - the bend that caught us out now has a chevron on it - I was navigating so can blame the driver for not taking the bend I'd called! Took us 20 minutes to regain the road but we ended up winning the event, but that's another story!
... I thought I would be able to overcome anyone using Google as it doesn't show (see pic), but Super Sleuth SteveH wins again! I like the explanation as well - I thought the pic was of a ghost, now it has been said then I can see a fisherman in so'wester. What we want to know now is how big was the fish!!!