SIngle elements do not prevent single elements from marching.
Sorry for this late answer Lawrence. As you can read in the previous messages of this thread, this is not the consensus of how marches have to be played. Toby Partdridge and Lorenzo Mele agree that single enemy elements do stop single elements to continue marching when within 400 p. Your interpretation was the first one I had when I read it first. Sadly, the wording is obscure enough to not bring light to it.
Toby said he didn't have his rules with him when he answered.
I think Lorenzo misunderstood the English, due to what he says (about contiguous single elements that are not lined up in a group) having been discussed during playtesting and possibly included in one of the draft versions.
THe text is "...all enemy within 400p are single elements or
are groups that (a)..(b)...(c)..."
That second "are" means that the (a)(b)(c) applies only to the groups, not to the single elements.
In my native opinion, that paragraph means:
Anything (groups or singles) can march (
i.e. start a march move) however they like if there are no enemy
groups within 400p (not counting any that are beyond friendly fortifications). Enemy single elements are not mentioned which implies they do not affect marching in this case.
Groups can march straight ahead if there are enemy groups within 400p, provided that all enemy within 400p fall into one of the following categories:
1. Single elements
2a. Groups that do not contain contiguous-non skirmisher elements
2b. Groups that are entirely train
2c. Groups that have a rear corner closer to the marchers than both front corners.
This would have been less confusing if it was worded "...and all enemy groups within 400p..." instead of just "...all enemy within 400p..." because then there would have been no need to mention single elements at all. Unfortunately we didn't think of this at the time.