The leaf looks as if it has been chewed by vine weevils, expat. All they are doing at the moment is disfiguring the leaves; this will not kill the rhododendrons. The main problem is the grubs that eat the roots of plants, particulary those of the primula family. Vine weavil (look for a picture on the web) are nocturnal and the best treatment for adults is to go out at night with a torch and pick them off. What you tell the neighbours is up to you ;-)

Thanks Bulbaholic, I'll have look....As my nearest neighbour is over a Km away I'll probably be OK.../.