What happens after the minor claim is served?

When the defendant receives the claim he has 21 days to decide what to do. During that time the plaintiff cannot do anything to proceed with the claim.

The three choices which the defendant has are:

Pay the full amount of the claim or come to some arrangement with the plaintiff to resolve the claim. If the defendant admits that he owes some or all of the amount he can fill in the front of the claim form and return it to the Court.
Defend the claim, with or without a counterclaim
Do nothing