I updated my post. Phil, I believe the issue that you are encountering is a result of using an ADK 8.1 based image instead of the ADK 8.0 image. You can determine this by checking OS version on your boot images. The supported boot image will state 6.2.9200.x instead of 6.2.9600 .x
Information of how to use an older PE to support XP Migrations:
http://davejaskie.wordpress.com/2014/04/03/installing-a-legacy-boot-image-in-sccm-to-support-xp-migrations
Updated Blog Post:
http://davejaskie.wordpress.com/2014/03/30/system-center-configuration-manager-sccm-2012-r2-and-xp-migrations-require-hotfix-2910552-to-ensure-no-ntldr-missing-error/