From what you've said I'd suggest that, if it does turn out to be a security based issue, the Autowatch immobiliser is the more likely source of this problem, simply because the Subaru transponder based immobiliser will allow the engine to turn over on the starter - it just won't allow it to start.
If it won't even turn over, the other obvious causes would be a duff starter or key switch - although one would presume that the very nice man will have ruled these out.
If it is security-related, the inference is that the non-factory fit immobiliser locks out the starter circuit, and that the immobilisation relay is past it.