Maybe this will help:

http://bulgier.net/pics/bike/Catalogs/Mercian-78/

Otoh, when I got my Mercian in '78, it was different from any in the catalogue at that time, and it was called 'Routier'. It was 'from the rack', so not custom build.

http://www.wooljersey.com/gallery/v/...tsen/album151/

So, as the catalogue says on almost every page: 'or to your own specification'!