Does any one know what the new road tax will be on the SDV8s or 5.0SC will be after this date? Thx.

I think the plan is to have a flat rate of £140 a year plus £350 a year for cars costing over £40,000, for cars first registered after April 2017, so £450 in total for new 405s. There is usually a budget in March, so this may yet change. At the moment there is a new car registration fee which takes it to £950, I don't know if there would be anything like that for post-April registrations. Only Range Rovers since 1988

I pay over £500 now so it will be better then?

Have a look at www.gov.uk/government/publications/vehicle-excise-duty.

Subject to any budget changes, the new rates seem only for cars first registered after April 2017, existing ones stay the same.
Having looked again I see that there are indeed first year charges on top of the ongoing tax. What you pay depends on whether you get a new 5.0 SC or 4.4SDV8, the former is very naughty and so the first year charge is £2000, the latter is in the next lower band and £1700. Whether that is "better" is a matter of opinion. Only Range Rovers since 1988

I believe the premium on top of the road tax is payable for 5 years, but I could be wrong.

Steve. 1968 Series IIA Pastel Green, nearly there project

Yes, the extra £350 for cars over £40,000 is payable for five years, the £2000 or whatever is the first year only. Only Range Rovers since 1988