Hi Neil, 

You would normally use  'y' when it replaces a specific place previously mentioned.

Tu vas à la plage ? Oui, j'y vais dans une heure = Are you going to the beach, yes I am going there in an hour's time

If someone just said out of the blue -

Tu y vas? 

My answer would be 

Où? 

as unclear to where we are talking about. 

Here is the Kwiziq lesson on how to use 'Y' -

Y = There (adverbial pronoun)

In the Kwiz question, we don't know what 'there' stands for, so we use là.

Bonne Continuation!