John,
How well has your car been maintained over the years? A 10 year old car shouldn't be a write-off, especially with such a low mileage. One question - is the timing belt due for a change... that can be a pricey job to get done. I don't know what 'pulling petrol' means, did they expand on that? Did the additive have a material impact on the readings in the NCT? At this point, I think the best move would be to seek a second opinion some where else. If you say where you're from, someone here might have a mechanic recommendation.
I'm amazed that cheating the NCT system is still possible, I thought that had been caught a few years back (when it could be done for just €10 in some parts).