I got this off of the the DVLA.uk site. But to make it easy for you, if the mini bus has more than 9 seat including the driver you must have a PSV lisence (D1) I hope this is of help to you.
Driving a minibus
You may be allowed to drive a minibus if it is not used for hire or reward. Hire or reward generally means any payment in cash or kind made by or on behalf of passengers that gives them right to be carried in a vehicle.
Driving licence held before 1 January 1997
If you had entitlement to drive cars before 1 January 1997 you can drive a minibus provided it has a maximum of 17 seats including the driver's seat and is not being used for hire or reward. Your minibus entitlement will remain valid in the United Kingdom until your licence is next renewed.
Meeting higher medical standards
When your licence is renewed, your minibus entitlement can only be issued if you make a special application, which will involve meeting higher medical standards.
If your minibus entitlement is renewed, at the age of 70, you will normally be granted a licence for three years that will allow you to drive a minibus not for hire or reward. If your minibus entitlement is not renewed, the entitlement will no longer appear on your licence.
Driving under the permit scheme
To drive a minibus that has nine or more passenger seats for hire or reward you will normally need passenger carrying vehicle (PCV) entitlement. To obtain this you must meet higher medical standards and take a further driving test. However, if you drive a minibus for an organisation under the minibus or community bus permit scheme you will not need to have the higher PCV entitlements.
Vehicle category descriptions
Vehicle category descriptions, which appear on your driving licence, can be found on the link below.
Drivers who do not have minibus entitlement
If your driving licence does not allow you to drive a minibus, there are certain circumstances where you still may be able to do so.