1. A method of processing payment authorisation requests for payment transactions to be conducted via a data communications network on behalf of online merchants, the payment authorisation requests being conducted as a result of orders by financial instrument holders via a plurality of different online merchant systems, each of the online merchants having an online merchant identity, and each of the online merchants being associated with one of a plurality of different acquiring banks, wherein the method is conducted by a trusted central intermediary system which is configured to transmit payment authorisation requests to each of a plurality of different online merchant Internet Payment Service Provider (IPSP) systems, each of the merchant IPSP systems being configured to transmit payment authorisation requests to at least one of a plurality of acquiring bank payment processor systems, each of the plurality of acquiring bank payment processor systems being responsible for processing payment authorisations for at least one of the acquiring banks, and the method comprising: receiving from a first online merchant system responsible for originating payment authorisation requests for a first online merchant, a payment authorisation request relating to authorisation of a payment transaction, the received payment authorisation request being initiated as a result of a financial instrument holder conducting an order via the first online merchant system; in response to receiving the request: a) generating a payment authorisation request comprising transaction data including: i) a financial instrument identity to be used in the payment transaction by the financial instrument holder; ii) an online merchant identity, associated with the first online merchant, as the payment transaction beneficiary; and iii) one or more transaction details including a payment amount; b) retrieving transmission data to enable the transmission of payment authorisation request data to a selected merchant IPSP system associated with the first online merchant; and c) on the basis of the retrieved transmission data, transmitting the generated payment authorisation request to the selected merchant IPSP system, wherefrom a further payment authorisation request may be generated and transmitted to an acquiring bank payment processor system responsible for processing payment authorisations for the acquiring bank with which the first online merchant is associated.