You missplaced the span opening tags before the questions if each field is present. Also there should always be one identical id at the current page, so change the parameter of the ids. (also change id="button" to class="button")
<h1 style="font-size:18px; color:#1e238b;"> FEATURES</h1>
<?php if( get_field('dab_radio') ): ?>
<span id="one"><?php the_field('dab_radio') ?></span><div class="button">DAB Radio</div>
<?php endif ?>
<?php if( get_field('bluetooth') ): ?>
<span id="two"><?php the_field('bluetooth') ?></span><div class="button">bluetooth</div>
<?php endif ?>
<?php if( get_field('air_con') ): ?>
<span id="three"><?php the_field('air_con') ?></span><div class="button">Air conditioning</div>
<?php endif ?>
<?php if( get_field('sat_nav') ): ?>
<span id="four"><?php the_field('sat_nav') ?></span><div class="button">Sat Nav</div>
<?php endif ?>
<?php if( get_field('heated_seats') ): ?>
<span id="five"><?php the_field('heated_seats') ?></span><div class="button">Heated Seats</div>
<?php endif ?>
