I finally managed to summon up the enthusiasm to go and see the GND this morning. After nearly a full circuit of the reservoir completed there was still no show and the sleet was coming down more and more heavily, it looked a bit grim to be honest. Then suddenly I caught sight of it on the far side of the water so I headed back round for a closer look. I managed reasonable views down to about 100 yards, just enough for my compact camera to record a few plumage details before it went back in it's case before short circuiting.
I've been birding now for 36 years without EVER seeing a GND so I've finally unblocked this bogey bird once and for all - maybe we should have a trophy for the longest standing (commonish) bogey bird!
Thanks to the finder and all those who've kept this blog updated with it's presence.