Well guys it worked but I had a bad moment when I thought it had not as I will now explain.
I did the speed tests and opted for 100M block size (15seconds). On my calculations this would take 3hrs 20 mins for the 80Gb disk. I used the dd command as discussed earlier and went away. After 4 hours the disk activity lights were still on and the disk were spinning. After 8 hours I decided that it must have all gone wrong. The cursor in the terminal window had not moved at all, so I killed the session and went away to have a think of plan B. A few hours later I ran up Knoppix (CD) again and after it had loaded I noticed that it was now showing two partitions on the new disk. On investigation I found that the files on both the disks were identical. So, tongue in cheek, I popped the new disc into the PVR turned on and waited for the error messages but to my complete surprise it booted up and worked ok.
So anyone any ideas as to why it appeared to hang ?. There were no bad sectors on the new disk by the way.
So a big thank you for all your help and comments once again.