Avi

If i buy a car off Mr X, and i have added the car to my existing policy today. what are the consequences, as he still has the car insured with his insurance company, and it won't be cancelled until next week?

Obviously if i crashed the car or whatever, i would claim against my own policy.

wrxsmo

As Mr X no longer owns the car the cover would no longer apply.
In the event of a claim I guess the insurer could possibly argue double indemnity (ie both refuse to pay as both covering the same car) but assuming the V5 shows the change of ownership & the date on inception of your policy as the same then there would be no problem.

Chris