From this short history, what are the likely diagnoses?
With acute onset and localising signs, a "posterior circulation" infarct is likely. Other diagnoses to consider include migraine aura, vertebral dissection or a space occupying lesion in or near the cerebellum.
What will be your initial imaging investigation?
A CT head will be the most likely investigation you will have access to. This can show if there is evidence of a stroke or other underlying pathology.
Sudden onset visual disurbance. Mild ataxia and loss of balance.