Rhododendron : Dark irregular spots on leaves

I planted a new rhododendron in my garden at the beginning of the year.  I planted it in ericaceous compost and my soil is acid in any case.

I've just noticed that the majority of the leaves have developed irregular shaped browny black splodges  .. when I look at the underside of the leaves they appear to be 'thinning' where the black spots are.

Has anybody any idea what this is and how to get rid of it ?  Previosrhodedendrons which I've grown in the garden have been fine although this is the only one currently growing.

Thanks,

Chris