It surely IS Plectranthus amboinicus, which is often called, amongst many other names, '7-in-one' herb or similar names. There's a green one and a variegated one. You'll often get smaller leaves when it's in a pot (especially if it's potbound), and they'll often be darker green than usual when kept in partial shade and well-watered. Nurseries often used high-nitrogen fertilisers to give a deeper, greener look to plants they want to sell in a hurry. It's a trick-of-the-trade to catch those who are shy of turning a plant out of its pot to look at the bits in the dirt.

Definitely not amboinicus - a plant which I've done a lot of research into and have grown both forms of. The leaves, stems, flower spikes and scent are markedly different.
The plant was not bought in a nursery, but on a charity stall. It was grown in poor soil to boot.

Andy - I did some research, maybe it is a cultivar of amboinicus? These plants have been in cultivation a long time and would have been selected for different characteristics in different areas, eg. all the many diverse forms of Thymus vulgaris. Just a suggestion.

Plectranthus argentia and Plectranthus fruticosa are lesser known herbs. I can't find any info on them at present other than they exist! Perhaps you have stumbled apon on of these?

Andy, may not be of any help, but maybe you have a plant that has been mislabelled which has probably been known to the seller by this particular name. Sometime ago I bought a plant from a school fete which was labelled 'plectranthus'..the leaves were similar to the spurflower or plectranthus saccatus but on flowering it had clusters of blue flowers. A search revealed it to be Eranthemum pulchellum (or Blue Sage). The lady who provided the plants to the fete was a delightful elderly person who had been gardening for years and probably had always called the plant by this name.

So in a roundabout way I wonder if this could have occurred with the plant you have.

I have some plectranthus growing which have small, dark coloured leaves with purple undersides. They have a very faint smell, though nothing like the '7 in one'.

OK! That narrows it down even more. I did think it could be a cultivar of P. amboinicus, but something tells me it's a separate species.

Ifound the following page for P. fruticosus:

http://www.plantzafrica.com/plantnop/plectfrutjames.htm

When my mystery plant is looking a bit more photogenic, I'll post a pic.

Thanks to everyone for all the help so far.

Very interesting Andy, keep us posted.

Andy, what market was that? I'd be interested in having a look if people are selling interesting and unusual things.

It was a stall in a "craft market" in Goolwa - not the actual Goolwa market itself. There were no other unusual herbs - I suppose I was just lucky.