The end of ye olde Tax disc..

[Niall]

Correct. It goes into the central treasury fund.

But where does the money come from to fix the roads?

Local council funding which yes is influenced by that central pot but only a set amount and not by how often cars use the road.

Fair point.

UK motorists paid £5bn in road tact last year. The government (apparently (you wouldn't believe it driving around here!)) spend £9.5bn on upkeep of the roads. Arguably, it would give more money to the 'fund' to allow increased spending. But thats an un-educated guess.

Well if you look at it that way, its swings and roundabouts. Yes you would probably put more money in the pot that way but then it would take more people off the roads.
Example, if i was self employed but had the car i have, thats free to tax yet if it was under this idea of extra tax on fuel, i would be f*cked considering as it currently is, my yearly fuel bill is about £4,000.
End of the day, there is guys in government who are being paid far too much who work this stuff out for a living who I'm sure know a substantial amount more than a few opinionated mongs on a forum