In a forum post concerning the Kingston Bridge there was mention of a concrete ship used for storage. I checked back on old posts and photographs and found this that shows a ship shape on the ground alongside the Brighton Road.
The location is the tar works yard immediately west of the old railway goods yard for the harbour. Another photo appears to show the same vessel with a smaller one alongside it. The smaller vessel looks very much like the seaplane tender RAF109 (built at Whitby, not Shoreham like a lot of other concrete craft) that tended the seaplanes at Shoreham, later purchased by the tarworks. The proportions of both vessels look to be similar to the length of the crete tugs built at Shoreham (125 feet) and the barges (170 ft) so chances are the storage tank was indeed one of the concrete barges.
It has been confirmed that the barge was Creteangle but was not shoreham built. It was launched at Hamworthy in 1919 and broken up at Shoreham in 1957.