No worries, thanks for this.
I've got two of these on the page. When I put the following in I get an error on save:
$fields->addFieldToTab("Root.Content.Images", new FieldGroup(
new TreeDropdownField("ContentTopImageLinkID", "<b>Top Image Block</b><br />Select the page to link to and then choose the image to display", "SiteTree"),
new ImageField('ContentTopImage','')
));
$fields->addFieldToTab("Root.Content.Images", new FieldGroup(
new TreeDropdownField("ContentBottomImageLinkID", "<strong>Top Image Block</strong><br />Select the page to link to and then choose the image to display", "SiteTree"),
new ImageField('ContentBottomImage','')
));
When I remove one of them it doesn't save the page id in the sitetree