Otherwise, the plant and the other fruits are fine, but I'd be interested to know what people think may have caused, & what I can do better to prevent?
:confused:

Sneezy,
This does not look like citrus scab to me, it looks more like wind scar. Scab does not usually affect only a couple fruits, but rather the tree as a whole. It will affect the leaves as well as the fruit. Wind scar on the other hand only affects the fruits that have rubbed against a twig or leaf. The rubbing may occur when the fruit is very small and show large later.

Below is a link showing citrus scab and what to do for it.

http://edis.ifas.ufl.edu/CH014

Your fruit does not look like this to me. You might take the fruit or the pictures to your local extension office for a better diagnosis.
Andi