Company Car change from Mileage allowance to running expense

[sjle]

I am a sole trader (and a newbie to Taxationweb) and I use my private car for my business. I am considering buying a new or nearly new car and would like to know the best tax option.
My trading year runs April to April. It would be preferable to buy this asset before the end of the tax year to make use of my AIA. If I buy a car before April 5th through my business, how do I account for the change mid way through a tax year? Should I be claiming the mileage done to date at the 40p per mile rate and then switch on a specific date to Capital allowance, depreciation and all running expenses?

Also, which date is considered the purchase date of the car, (I am not sure when I would be invoiced if delivery is a few weeks).

To make matters a little more complicated, I am planning on setting up a limited company after April 2010. How would I treat the car and any other assets that are brought into the business and how would I value them (car, computer equipment, other equipment, stock)? If the car was a new one below 110g CO2/km, presumably it would have a WDV of zero on 6/4/10?

Many thanks in anticipation of your help.