Add the duty to fuel. That way I would not have to pay when the vehicles are not being used. At the moment in my house we have the following:
My car - Used 4-5 times a week at the moment but paying £250 (ish) a year for tax so I can park it in the road.
My van - Used once a week on average but paying £250 (ish) a year for tax so I can park it in the road.
My bike - used once a month on average but paying £80 a year
Wifes car - used daily paying £200 ish a year
Sons car - Used fec all as he is at university but paying £250 (ish) a year for tax so I can park it in the road.
Sons bike - will probably get SORN when it runs out as he is away at Uni and we keep it off road.
Other years I will do 20,000 business miles and accept I would pay more for tax but the fairest way is to pay as you use. The government made a big thing about car tax being related to emissions. Fuel consumption is directly related too really. The more fuel you use, the more you pay.