Due to other commitments, I left it a long time to follow up on this, too long as it happens!
Dell told me late last year that, since Windows XP support has ended, they no longer supply disks for XP. I had expected that they would still have ISO images of the disks and could burn one on request. However, they had not kept copies in-house for a couple of years but instead had been shipping the disks directly from Microsoft.