Have you change fuel supplier? A combination of a tired catalytic converter and supermarket fuel can cause this effect, according to some web reports on other car forums.

I usually use Shell V-Power (think its actually 100ron now) and in Malvern there are a lot less Shell stations and an abundance of BP, so I was using BP Ultimate 97.. could well be this, would love to know the science behind it though.

After reading this thread I did notice an ammonia smell from the tailipie of my Merc diesel. Sure it wont be the same chemistry but the relevance to the OP is the possibility of the smell being from the exhaust which in turn would suggest some sort of exhaust leak since the smell is coming from under the bonnet.

The smell having gone away doesnt mean any leak, if any, will also have gone