Niall,
Here is a screenshot of the task sequence. I'll grab a copy of the smsts.log file and upload it shortly.
It looks like the task sequences provided by HP, Dell, and IBM are very similar and I found some information on Dell's site related to troubleshooting issues with variables. They provide a sample script for troubleshooting. Here is a link that discusses the issue.
http://en.community.dell.com/techcenter/os-applications/w/wiki/2554.aspx
Doug