Having looked around at a few "how to build trees" tips on numerous websites (and there is some excellent advice around) I thought I would experiment.
One site suggested heather or sage bush so during a walk up Moel Famau I gathered a few bits of broken off heather. A few choice bits went into the microwave (as advised) for a few minutes, sufficient to kill any bugs or fungus but not enough to set them alight. (The following picture is a serving suggestion only)
The bits were super glued together, roughly copying an ash tree.....
And a coating of Display Mount and Noch leaves to finish.
Time consuming, but I am happy that this looks much better than earlier attempts. Mrs Chas suggested using dried herbs instead of Noch leaves. What a good idea! I shall give it a go. The next challenge is a generic conifer.
Addendum: Geoff's comment below includes a link to one of his typically detailed master classes, this time on model trees. It's a much better description than here!