Service contract terms and conditions state:
"The Customer has the right to end their contract with MercedesBenz Cars at any time. If the Customer ends the contract before
its expiry, other than due to a breach of the contract by
Mercedes-Benz Cars, the Customer shall be refunded all
payments received from them, less any costs incurred for
servicing work already completed and minus a reasonable
administration fee of £40 plus VAT.
On the ending of the contract, however that may occur, the
Customer shall remain liable for all sums outstanding at the point
of ending the contract for services completed before the ending
of the contract by Mercedes-Benz Cars or Retailers. The
Customer shall make payment immediately of any such
outstanding sums on the ending of the contract."
Phone them up and ask them how and why you are in arrears. It is pointless us speculating.