Good news - i have just called Dell Technical support who have been extremely helpful. They had no problem speaking to me and have agreed to send me a replacement disk at a reasonable cost, so it would appear that they are still distributing XP disks. Just thought I'd post that in case it helps others in the same boat.
I bought an Inspiron 510m laptop brand new from Dell some time ago, and am now installing a new hard drive. However, although I have the Windows XP licence key on the sticker on the laptop, I don't have a Windows XP installation CD - would Dell please be able to send me one and let me know who I should contact with my details ?  Many Thanks, 
Windows XP Disc required for new hard drive install