My problem is that ASP.Net does not include the necessary CSS tags to distinguish the selected items on the menu.
I have since developed this "workround" - but don't really understand why it is necessary.
protected void Menu1_MenuItemDataBound(object sender, MenuEventArgs e)
{
SiteMapNode smn = SiteMap.CurrentNode;
string item = e.Item.NavigateUrl;
bool Selected = false;
if (smn0 != null)
{
Selected = smn.Url.Equals(item, StringComparison.CurrentCultureIgnoreCase);
}
e.Item.Selected = Selected;
}