Hitachi, which provides a management service with maintenance, repair, accident and driving licence management for Magnet's 400-strong fleet, initially agreed on a two-year contract, but this has been extended for a further three years.
Since the contract started, Hitachi has focused on fuel management and pooled mileage systems to cut costs.
Nik Browne, HR services manager, said: 'We are not a straightforward account so we appreciate the fact we get a great deal of flexibility on customer service.
'We are making cost savings as a result of measures introduced over the last two years, which have made a big difference.'
Hitachi offers several funding methods to Magnet including contract hire and a personal leasing scheme for drivers not using company cars.