To be honest, all the Northumbrian Water reservoirs are upland waters and all are prone to be choppy at the best of times. The thing with Derwent is that, while it does have a fly only bank, that's the one that normally faces into the teeth of the prevailing wind, although there is a very short length of bank that's usually shelterd at the mouth of the 'bay of plenty'. If you fish on the multibait bank, you'll normally have the wnid at your back, or at worst blowing accross, but you will also have bait anglers along all the most accessible parts of bank, even on weekdays.
At Grassholme, which is also fairly popular, you'll probably have the same problem, only on both banks. Ditto the other stocked fisheries at Fontburn and to a lesser extent Kielder.
Maybe
Hury, which is both stocked and fly only (and also quite close to Grassholme), might be you're best bet.
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