That looks like scab Ben.
Spraying with sulphur or a commercial product for scab control will keep the tree clean.
The sprays stop the spores of scab from germinating, so they are protectors ...not eradicators.
This means that you have to keep a regular spray programme going during the growing season so that new leaf gets protected as it grows.
The spores spread in warm, damp periods known as Mills Periods.
Scab will effect the fruit as well, causing brown scabs to form on the skin of the fruit. It's only skin deep though and the fruit is still fine to eat.
Pears can be attacked by it too.