Chickenfish - Ive just got back from a week in Keswick, had some fishing planned but the weather beat me at every turn - we had a lot planned so I had to be pretty strict on the days I planned to go out and it was pretty much unfishable on those days with gales and driving rain (and it takes a lot to stop me fishing!). Gutted but ill be back.
From my research prior to going and what I saw you could fish the Derwent that runs through Keswick, start at the bridge in the centre of town and work your way downstrea, see
Keswick Angling Association, Cumbria The Lake District for all the details. As mentioned above Watendlath Tarn is about 20 mins drive from Keswick and is a lovely spot, if you fancy a bit of of wild fishing you can walk from Watendlath to Blea Tarn (takes about 40 mins and is a fair hike), supposed to be good wild brown fishing.
I managed to get up to Watendlath one evening to see what it was like for the next trip, the photo below shows how nice a spot it is (cost is £23 4 fish 9-5 ticket, £18 4 hour 2 fish ticket):