Engine won't idle but has good power when under way

My wife has just bought a 2001 Holden Vectra 2.2 automatic GL. It goes well but is idling very roughly and making a tapping sound. The Vectra has plenty of zip and everything works OK. However it is idling very roughly, hesitating and missing but it has never cut out altogether. It also makes a distinct tapping noise, which obviously would be tappet noise I assume.

We have had the car for three weeks now, and it hasn't let us down. Would a tune up such as new spark plugs and injector clean fix the idling problem do you think? As far as the tapping, if the tappets need repair/replace or whatever they do would that be a major and expensive repair. Regards.