It is a single-lane highway bridge, operated without traffic lights. It consists of two metal lattice girders at the edges of the deck, each with two planes of lattice web. The bottom chord of each truss, and any crossbeams, are encased within a concrete deck slab.
The superstructure is supported from a series of cross-braced metal trestle piers. These have inclined legs on the upstream side to protect them against impact from river-borne debris.
The metalwork is riveted, which dates the original bridge construction to 1960 or so at the very latest, and probably much earlier. A bridge of broadly similar construction at Heatherslaw Mill in Northumberland dates to the 1870s.
Update 4th September 2014:
The comments to this post offer further information on this bridge, and see also pages 206 and 207 of http://www.drakkar.co.uk/ch13.