Alignment and distribution doesn't work.

[rich2005]

[quote]...snip... Basically I have all elements as separate layers, or groups of layers, there is a top piece in a fixed distance from bleed edge, several elements to the bottom of it and the final elements on the bottom of the work area above the bleed border, the idea is that the top and bottom elements stay in their places and the elements between them are evenly distributed vertically with identical spacing between each element, at least rounded up to whole pixels. ...snip...[quote]

Not so easy to visualise, especially since layer groups are involved, you could post a typical .xcf  (dropbox or similar) for comments or it might be that your manual calculations are the only way.