To hire the buses you need to find someone who is willing to drive and is eligible to be insured.
Drivers need to:
be 21 years old or over
have held a full UK licence for three years or more
have had no accidents, claims or convictions in the past three years
have completed a driving proficiency test with the Minibus Manager
be a current RVC student
Insurance is only provided for the specific date requested each time, so even if someone has driven the minibuses in the past you need to request insurance for every date, otherwise you will be driving uninsured.