Looks like a long exposure to get some cloud movement and this silky smooth, very beautiful sky. The composition is good too, but to me it feels a bit overcooked in processing, the lighting looks a bit unnatural.
Cambridge from the Backs - and nicely captured, too!
edit - I see I was wrong with the location, but that doesn't take away from my comment - a pretty scene very nicely photographed. Another place to add to those I must visit!
Great photo, too bad I can see the where the layer mask ends for the sky (especially around the two bare trees to the left of the tower). Next time, try zooming in closer and spending more time going in and around each tree branch. It may take longer, but it will appear more natural looking (as natural as an HDR image can look :P). Great color in the sky as well. 6.