Thank you for your help - I now have a solution ...in case it helps someone else.....
...I also think I may have been a bit silly...
The solution - thanks to a great guy in Apple Support USA - thank you Matt!!
- first I partitioned my hard drive using disk utility - and set 235GB for a separate install - I called it Tiger install disk
- this allows your hard drive to act as 2 totally separate systems (by holding down the option key when you start the computer you can choose which you want)
- then I reinstalled Tiger from my original discs onto the new part of my hard drive
- then I read my backed up home folder and copied it to the Tiger system on the new hard drive section
- then I rebooted and started up Snow leopard on the original part of the hard drive
- then I used migration assistant to transfer from the 'Tiger install disc' drive - it was recognised by migration assistant
Where I think I may have been a bit silly
- I am now not sure if the orginal backup was always visible in Snow Leopard
- in Tiger I found backup.sparseimage - which when double clicked opened up my home folder
- now that I have istalled Tiger as well - I can also find this in now Leopard... ...so maybe I just hadn't opened all the original folders on my backup disk...
- ...but for sure migration assistant could not read the backup copy