Finally got around to doing this. I took out the hard drive from the old machine and using a little plug in usb adaptor, really neat, £10 from ebuyer, and clonezilla duplicated it on the new machine.
The problem however, and this has happened before with debian, is that xorg now seems impossible to configure if it doesn't work, except by editing xorg.conf by hand, which is not my favourite pastime.
Seems it used to be possible to use dpkg and reconfigure xorg, but if its still possible I could not find a way. So I started up from a PCLinux live CD, became root, set up the screen, then copied the xorg.conf file over to the debian installation, started up from the hard drive, and it all works.
This an i3 running on an AMD64 image. I am really a bit surprised that it works. But it seems to. Famous last words. Clonezilla is very easy, and reasonably fast considering the size of the partitions.
Peter