Heating the plug will make it slightly expand. Might want to freeze the plug. Put some oil on the hole if there isn't any there now.
You may need to remove the exhaust manifold it you can't get it in.

Possible bad engine temp sensor? If it think the engine is warm when it's cold the ECU won't know it needs to inject more fuel which is required to start a cold engine.
How does it run and start when it's warmed up?

Have you checked out what the fuel pressure is?
It's possible the pressure is low when you first crank it and it takes a few seconds for the pump to build up the correct pressure.
Try turning the key on but not cranking it, then shut the key off and then back on. Do this a few times before you try ...

What's the chance it could be the heat shield around the catalytic converter?

Typically if you tap the CAT with a rubber mallet the loose honeycomb inside will rattle.
I would think the CAT would rattle more when cold than hot.
Only a 3 year warranty on emission parts?
We get 3years on the car, except for the emission parts, which is 8 years or 80K miles IIRC.
Sounds like it ...

Not sure what your warranty on emissions components are in Austraila, here in the US they are 8 years (IIRC) so maybe the dealer isn't being 100% honest with you.

What if you have your foot on the brake, not hard, will it still make the noise?
If it still does you can rule out the brakes. Have you looked at the sway bar links?
I bought the Chassis Ear a few years ago, got it on sale for $99 USD.

Could it be the insides of the CAT's rattling?
The microphone tool I have is called the "Chassis Ear", it comes with 6 microphones, a box to switch which mic you want to listen too, and a set of headphones.
They have the wired and wireless mic version. Wired is less expensive, but you need to tape ...

You don't want to buy them from the dealer.
I use NGK Platinums in one car and NGK Iridiums in the other.

On my wife's '01 S60 and my '02 V70 the timing belt replacement is at 105K, so at 60K it doesn't need one.
When you take it in for an inspection if they can't tell you anything about the paint take it to a good body shop. Those guys can spot a repair and respray from a mile away.