An on-line source called RL Shep sells old pattern books. Their website is http://www.rlshep.com/HTML/booklist.htm

Their title "GREAT WAR Styles & Patterns of the 1910s" has a pattern for parade and mess trews for Scottish regiments, and a long description for when and where they are appropriate for wear, etc...

They are also a phenomenal resource for other historical patterns, & a lot of re-enactors use them.

Hope that helps, Ron