Additionally, you should reflect that there's been an error in the <title> by prefixing the existing title with "Error: ". That will make sure screen readers are alerted to there being an error as soon as possible.
Optional description of the errors and how to correct them
<div class="error-summary" role="alert" aria-labelledby="error-summary-heading-example-1" tabindex="-1">
<h2 class="heading-medium error-summary-heading" id="error-summary-heading-example-1">
Message to alert the user to a problem goes here
</h2>
<p>
Optional description of the errors and how to correct them
</p>
<ul class="error-summary-list">
<li><a href="#where-do-you-live">Descriptive link to the question with an error</a></li>
</ul>
</div>
<h1 class="heading-large">Where do you live?</h1>
<form>
<div class="form-group form-group-error">
<fieldset aria-describedby="where-do-you-live-error">
<legend id="where-do-you-live" class="sr-only">Where do you live?</legend>
<span id="where-do-you-live-error" class="error-message"><span class="sr-only">Error: </span>Choose an answer</span>
<label class="block-label" for="radio-1-error" data-target="radio-1-error">
<input type="radio" id="radio-1-error" name="radio-group" value="Northern Ireland" aria-controls="radio-1-error" aria-expanded="false">
Northern Ireland
</label>
<label class="block-label" for="radio-2-error" data-target="radio-2-error">
<input type="radio" id="radio-2-error" name="radio-group" value="Isle of Man or the Channel Islands" aria-controls="radio-2-error" aria-expanded="false">
Isle of Man or the Channel Islands
</label>
<label class="block-label" for="radio-3-error" data-target="radio-3-error">
<input type="radio" id="radio-3-error" name="radio-group" value="I am a British citizen living abroad" aria-controls="radio-3-error" aria-expanded="false">
I am a British citizen living abroad
</label>
</fieldset>
</div>
<div class="form-group">
<button class="btn btn-primary btn-arrow">Continue</button>
</div>
</form>