The Defendant either has to acknowledge service within 14 days of service of the claim or submit a defence within 14 days.
If the Defendant files an acknowledge of service within 14 days they will have a full 28 days from the service of claim within in which to submit a defence.
If the Defendant fails to carry out any of the above action within the timeframes required by the Civil Procedure Rules (as noted above), then as a Claimant you can apply for judgment in default.
This basically means you will obtain a judgment for your claim without the need for a hearing.
If your claim is for a specified amount you will obtain a judgment for this amount.
If it is unspecified, it may be necessary for a hearing in order for the court to assess how much you should be awarded.
Please be aware that a Defendant can apply for any judgment in default to be set aside although this is no guarantee that such an application will succeed.
Applying for judgment in default is govered by CPR Part 12 - Click here for a link to CPR Part 12