This is getting stranger by the minute.
I checked that the atrs.tar file is in the correct location in BASE. To make sure that the bootwiz process was copying it i edited the .ini file and replaced the %PATH_BASE% variable with the absolute path to the file. After this the whole thing started working. I put the file back to how it was, regenerated the preboot files and it is still working!
Since everything is working again i can get on with imaging machines, but i hate it when there appears to be no rhyme or reason to how something gets fixed. If anyone has a clue what could have happened i'd appreciate an explanation but its no longer urgent.
thanks
David