I'd second Kielder. Its my favourite NWL water. Great surroundings and loads of space. As 2604 says, it can be hard work though, I think it has the lowest rod average of all the NWL waters. Try Beeches Walk at Leaplish, Bull Crag (mainly in the summer) or down from Tower Knowle towards the Dam Wall. I think Hawkhope also fishes well, but I've not spent much fishing time on the north shore.
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Northumbrian Water -
Just one other point, it can turn into the North Sea given the correct weather conditions. Not a water to visit on a windy day!