In reply to mockerkin:
Pedants corner:
People would be wrong to say there is only one "lake" in Cumbria. There are obviously many examples of a "large, landlocked, naturally-occurring stretch of water", which is a pretty decent definition of "lake". Thus the correct answer to the question is actually the one that is intended to be the red herring, i.e. the number of the existing bodies of water that are big enough to be a lake (debateable given Brotherswater, Rydal etc?).
For the answer "one" to be valid, the question would have to ask about "Lakes". As the question is usually verbal, this is not expressed and the multiple answer is not wrong.
Anyway I was brought up in Whitehaven and the most common local usages I encountered there were "Ennerdale Lake" and "Wasdale Lake" so maybe it is only a matter of time until there are more "Lakes" anyway as usage seems to have influenced Bassenthwaite?