We'd pay a lot more for a Mk2 in New Zealand. An immaculate Mk2 45 is about £1200, so a diesel would be even more. It might be a bit late in the vehicle's life to export it, though
If it's truly in immaculate condition with a long MOT then it could go for between £400 - £500 I reckon. Only way to find out is to bung on Ebay and see what it fetches.
Thing is, a lot of people go for diesels because they think they are more economical than petrol. But with the Unos there isn't a lot of difference between the diesel and the FIRE engined versions in terms of economy, as shown in the official Fiat handbook! When you take into account the price difference between diesel and petrol the diesel version doesn't work out cheaper to run. However, as it is more torquey then it is better suited for towing if that's what a Uno is being used for.
But most people won't know this and will buy it for it's alleged superior economy