@MightyMouse (and anyone else who is interested too)

Although it is more expensive to change once you have taken at trip via national rail - it is cheaper if you just go point to point. This means that although its more expensive if you go to London Bridge via Southern and then change for the Northern Line to Moorgate, if you get off at London Bridge (same zone) it is 10p cheaper than taking the overground via Canada Water.

I agree, its very confusing.