To be fair, and I'm certainly no Brown fan, I doubt whoever is in power will make much difference to fuel duty levels. Like booze and tobacco, it's a popular target for duty rises because they're all relatively price-inelastic. They clobber them because they know it works. We moan about it for a while, cut back for a few weeks and then go right back to where we were before. And repeat, a few months later.
A change in government might help exchange rates long-term though. Maybe.