Hi Nicola watts,
A confirmed unilateral Optic nerve
enlargement should not be ignored and needs a detailed evaluation. It could be physiological or due to vascular disorders or neurological disorder(since you have a family history of MS) .
There are specific tests to assess the optic nerve function and without a direct examination it is hard to comment further.
So don't miss your appointment and go ahead with radio imaging investigations,if advised by your ophthalmologist( preferably a neuro ophthalmologist) to find the cause for your optic nerve enlargement.
Take care and do let us know after your consultation.