In Colin Magg's book "The Exeter and Exmouth Railway" published by the Oakwood Press, he mentions the rebuilding of the 114 yard long River Clyst viaduct on the line just south of Topsham. The old deck needed replacement, and the never before used piers that had been built for possible double tracking were utilised for the new replacement deck. The first train crossed on 18th December 1960. The old deck was then removed in 1961. There is no mention in this source of the deck's origin.
In the same book, there is also a photo of Warship no. 858 Valorous on bridge 15 at Exton with the Swindon 75 ton crane, on Sunday 16th November 1969. This is stated to be in connection with the laying of a North Sea gas main across the river bed. Could this be what you're thinking of, Mr Cowley?