Andrew,
According to British Aircraft 1809 – 1914 by Peter Lewis they also constructed the engine for the aircraft, so your suggestion that it was Charles Edward Metzgar looks likely to be correct.

There is an image of the aircraft here

http://www.shorehambysea.com/shoreham-historical-photos/sussex-archaeological-society-photo-archive/sussex-archaeological-society-photo-archive/shoreham-by-sea-aviation/a7-3187.html