(1) Where a person (in this section referred to as the claimant) claims to be entitled to recover from the owner of a mechanically propelled vehicle or from a person (other than the owner) using a mechanically propelled vehicle (in this section referred to as the user), or has in any court of justice (in proceedings of which the vehicle insurer or vehicle guarantor hereinafter mentioned had prior notification) recovered judgment against the owner or user for, a sum (whether liquidated or unliquidated) against the liability for which the owner or user is insured by an approved policy of insurance or the payment of which by the owner or user is guaranteed by an approved guarantee, the claimant may serve by registered post, on the vehicle insurer by whom the policy was issued, or on the vehicle insurer or the vehicle guarantor by whom the guarantee was issued, a notice in writing of the claim or judgment for the sum, and upon the service of the notice such of the following provisions as are applicable shall, subject to subsection (2) of this section, have effect:

(a) the insurer shall not after service of the notice pay to the owner or user in respect of the sum any greater amount than the amount (if any) which the owner or user has actually paid to the claimant in respect of the sum;