Instead, he decided to build an extension to link up with the Neath Canal basin at Aberdulais. Again he sought support from local landowners, including Lord Jersey, Lord Dynevor and the Duke of Beaufort, but again none was forthcoming. He decided to build it as a private canal, without an Act of Parliament, and work started in 1821. Engineering problems were experienced near Neath Abbey, where a cutting was required through what appeared to be quicksand. Eventually, an inverted masonry arch had to be built to contain the canal and stop the sand collapsing. The lack of an Act to authorise the canal proved to be a problem in April 1821, when L. W. Dillwyn refused permission for Tennant to cut through his land to pass under the Swansea road. In February 1822, Dillwyn obtained an injunction against Tennant, who then attempted to change Dillwyn's opinion by sending a stream of important people to argue his case. Finally, in the autumn, Tennant offered the Neath Canal terms for the use of the junction which were so favourable to them that they accepted. Dillwyn, who was a Neath Canal shareholder, was sent a conciliatory letter and eventually agreed to negotiate with Tennant, whom he described as "that terrible plague Mr. Tennant." The final section included the only lock on the main line, which was followed by a ten-arched aqueduct across the River Neath, and the junction with the Neath Canal. The total length of the canal, when it was opened on 13 May 1824, was , and it had cost around £20,000, which did not include the price of the land or of the harbour at Port Tennant.