changing motor insurance

[wendyl]

In July I changed my Motor Insurance policy to the Post Office. In August I arranged the purchase of a new car to be collected today. I contacted the PO on 18th August to arrange insurance changes and issue a new Certificate to organise transferring a personal number plate to the new car. Since then they have said paperwork has been sent again and again but has not been received until Thursday this week too late for this to be processed. Another phone call to them today regarding keeping my present car insured as I cannot collect the new one until the change of plate is confirmed, has been unsatisfactory.
Can anyone advise if I can cancel the insurance and move to another company at the time of eventual delivery and the best way to go about this.