I got hit too. I switched gas and electricity at the beginning of December. For gas, I've only made one advance payment so far, but for electricity, I've already made two. The other way around would have been preferable, because now I'll have to make an additional payment for gas and I should actually get a refund for electricity, which I probably won't receive.
Can I actually refuse the additional payment for gas on the grounds that I would still get something back for electricity? Probably not, right? They are two separate contracts... :(