What's it for, the car I'm guessing?

Best bet is any kind of cheap foam packs, you should be able to find bargain bin stuff on ebay (edit: or the NZ equivalent).

That dynamat stuff seems popular, been around since my cruising days  it's not 'cheap' but nor does it appear to be overly expensive 

It is here.
On Us sites people using "Peel & Seal" but could not find similar. Would the silver foil based lamented flooring underlay work? Thats supposed to make walking on it less noisy?

Car underlay from a scrappie seems like a good bet from reading a few other forum's threads,  or rubber backed carpet tiles, also flat roof insulation, the sort you get from B&Q, stuff like Aquaseal or flashing tape. Most people seem to say its worth spending a bit of extra money on Dynamat or similar.

Dynamat is simply a foil layer with rubber underneath it which has a sticky side. If you google Peel and Seal stuff that is what people say it is. Flashing tape I heard does the job. I think I will go look in Bunnings or/and mitre 10 next weekend when I get my car back.
(they basically two types of B&Q but bigger, NZ and OZ's are way more into DIY.

I think its crazy throwing money into sound deadening, your not going to get it back, i've had some big installs years ago and i never bothered with deadening, your better off channeling the cash into a better headset, components, amp / sub, better cabling or split charge system, at least you can take it out and sell it all when you get bored of it.

Got a great headset and new front speakers. The FTO is a great car but has minimum between the metal and the interior. The boot is something that is a must to do along with around the speakers in the door frame. While it is cool to hear the engine you really here the engine as well. I have heard before and after work on the FTO and even simple work makes a huge difference. The car had great rear speakers when I got them, I thought they were blown but they are not. The reverb in the boot simply makes them sound bad.