I resolved the issue -the problem was the <div class="container"> which was wrapped around the footer-right block. It has a default style applied in the template which forces it to stretch full width (more or less). I removed that from around footer-right and now it works -I now have the single row split into 2 equal blocks. This is the amended code in case it helps someone.
<!-- FOOTER -->
<footer id="t3-footer" class="wrap t3-footer">
<div class="container">
<div class="row">
<?php if ($this->countModules('footer-left')) : ?>
<!-- footer-left -->
<div class="footer-bw-l t3-sl t3-sl-1 col-xs-12 col-sm-3 col-md-6<?php $this->_c('footer-left') ?>">
<jdoc:include type="modules" name="<?php $this->_p('footer-left') ?>" style="raw" />
</div>
<!-- //end footer-left -->
<?php endif ?>
<?php if ($this->countModules('footer-right')) : ?>
<!-- footer-right -->
<div class="footer-bw-r t3-sl t3-sl-2 col-xs-12 col-sm-3 col-md-6<?php $this->_c('footer-right') ?>">
<jdoc:include type="modules" name="<?php $this->_p('footer-right') ?>" style="raw" />
</div>
<!-- //end footer-right -->
<?php endif ?>
</div><!-- //end row -->
</div> <!-- //end container -->
</footer>
<!-- //FOOTER -->