Hi Graeme

The disease in your Silver Beet looks like Cercospora Leaf Spot and is a common fungal disease. Apparently it is most common in warmer climates with higher humidity. It can be treated with a fungicide, but I wouldn't recommend this - if you remove infected leaves as they appear, and regularly pick the leaves (don't allow old leaves to remain on the plant) you should find that you will still get a good crop.

Word of warning ..... don't add infected leaves to your compost! Also, ensure that you practice crop rotation (see article on iCultivate) to minimise the problem.

I have provided link that contains further details that you may find useful.

All the best.