I updated the motherboard to the latest BIOS, v1011 (beta), however the problem remained.
Further digging turned up Bug #213754 in linux dell xps 410n won't reboot - regression at https://bugs.launchpad.net/ubuntu/+source/linux/+bug/213754 which appears to be the same bug.
Following the instructions in that bug report I have found that the reboot=b workaround works on my system.
I updated the motherboard to the latest BIOS, v1011 (beta), however the problem remained.
Further digging turned up Bug #213754 in linux dell xps 410n won't reboot - regression at https:/ /bugs.launchpad .net/ubuntu/ +source/ linux/+ bug/213754 which appears to be the same bug.
Following the instructions in that bug report I have found that the reboot=b workaround works on my system.