British Gas has two ''Fix & Fall' offers, which Bobs561941 seems to have confused.
Firstly, he can choose to sign up to the 'November 2014' deal, which guarantees that the prices which will apply from 23 November won't rise for a full year. Those prices aren't mentioned in the letter Bobs561941 refers to; he need to look at the other document, which came with the letter, to see what they are. (If they're the same as mine, they'll be 4.73p/kWh for gas and 12.73p/kWh for electricity).
Secondly, there is a 'November 2016 Fix & Fall' offer, where he is offered the chance to pay a supplement of 0.28p/kWh on gas, and 0.84p/kWh on electricity from 23 November onwards but with the guarantee that those prices won't rise for 3 years.
If he accepted the latter offer he'd be paying 5.9% more for his gas (and 6.6% more for his electricity) than most other British Gas customers for the forthcoming year. However he'd be paying roughly the same as others (but probably slightly less than them, if we can assume a similar price rise in 12 months time) in the second year. He'd then quite possibly be paying significantly less than other BG customers in the third year.
If it was possible to assume that the price of fuel will rise by about 10% next year, and by 10% again next year, the 'November 2016' offer represents quite a good deal. However if the Government removes some, or all, of the 'green tax' element of forthcoming price rises, then BG might be able to keep their increases lower over the next 2 years.
Since all fuel companies charge different rates in different areas, and we've got no access to the prices charged by either BG or NPower where Bobs561941 lives, it's impossible to say whether switching to NPower would make sense.