Ok, I think I have cracked this one... I used the Ext2Fsd project to install drivers for Ext2/Ext3 drives and I can see the drive in Windows Xp and can copy it off the drive onto my NTFS drive...
This is what I did:
1. Using my WD external USB drive I attached it to the Humax (front port) and got the Humax to format it, it uses Ext3 as the format, copied a file that is over 4GB to the drive, Humax can see it fine.
2. Downloaded the Ext2Fsd software from:
http://ext2fsd.sourceforge.net/projects/projects.htminstaled it, got it to start as a service and automatically assign drive letters, Rebooted my XP machine
3. Booted with the USB drive attached, opened Explorer and the Ext3 drive is there as per any other drive and can now copy to and from it as per an NTFS / FAT32 drive.
I will test this more by putting the drive back on the Humax and copying a whole set of files across and then see tomorrow if they come across but am hopeful that this will work.
Cheers,
Scott.