Up to 2006 you dont need a diagnostic tool,just press and hold the LH trip button while you turn the key to position 1.After about 5 secs the display will show SIA reset,from there you are on your own as I cant remember the exact proceedure,I do have it written down on a document from LR at my workshop.I think you then release and then repress that button for a further 5 secs - something like that,you'll just have to fiddle around till you get the right combo.You cant do any harm.
LR never got round to doing the software to enable Testbook to do it.