Harry,
I don't know anyone who has contest the fine regarding catalyic converter (others might), for this, sorry I can't be more help to you. Might try to contact the mechanic who did roadworthy or previous owner and see how they feel about it (doubt if they care but at your situation, it is never hurt to look at all options).
After quick read on VSI26 (Roadworthy requirments) states that all exhaust and emission controls that the said car are equipped are to be there when the inspection is being taken place, I suspect the previous owner chucked the standard exhaust system to pass RWC then swap back (pretty common).
You could use this to agure your case if you were to 'lawyered up' (talk to solictor/lawyer/legal aid about this and see what they say), Write a well written letter explaining your situation or call/go there and discuss first before the next step further (this is what I would do).
Again, hope it helps. 'R31Nismoid' could help you as he know about EPA and such better than me.
Cheers
Grant