I bet the guy at NPower, that thought of that little brainwave, thinks he's a right clever little ........... !
I bet that 90% of the population (which more than likely would include me! ), would have failed to pick up on this. I'm not a NPower customer, but full respect for noticing this, as I'm sure other companies will try the same.
Just another side to this, I'm not with powergen but do pay monthly. Our company (npower)always over estimate in the summer and under estimate in the winter. I tend to update the reading via their website whenever estimates come through.