So ,is it actually ticking over too fast ,or is the rev counter inaccurate?

I would take it to a different dealer for a second opinion as it doesn't sound right to me.

The rev rise at first seems counterintuitive - any aircon effect I've noticed on my cars has been a slight drop. It would irritate me if it rose. However I think mss1tw has hit the nail on the head in terms of how the 500 deals with the power issue and the rev rise may be normal in this instance.

I've done a couple of minutes googling and there are indirect references on Fiat sites to 500 and other Fiat diesels reving higher at idle when aircon is engaged. None suggests its a problem but they don't go into much detail - one suggests the software adapts to your driving style and settles down after a few weeks. Sounds unlikely to me though.

Sorry I couldn't save the links but Marko might find out if its a real problem or an annoying case of "they all do that" if they search the specialist Fiat/500 forums.