A couple of my courgette plants have become stunted, and have deformed leaves, what could be causing this?

Your courgettes have been infected with mosaic virus disease.

The virus enters the plant through the sap, and is spread by greenfly when feeding.

I am afraid there is no control, it would be best to remove them before they affect adjacent plants.

Is there an effective cure for whitefly on courgettes?

White fly are becoming immune to a number of the insecticides available today.

To clear up an attack spray weekly with various white fly designated insecticides.

Each week use a different type, as this tends to weaken them and overcome the immunity.

Ensure you coat the underside of the leaves with the insecticide.