I found this thread which is really close to what I need, but not quite. It has this code:

(function() {
  window.addEventListener("load", function(e) {
    document.body.innerHTML = document.body.innerHTML.replace(/<((?:b);)/g, '&$1');
  }, false);
Which turns &amp;lt; into &lt; and &amp;gt; into &gt;. My problem is the opposite as I require &lt; to be turned into < and &gt; to be changed to > as the HTML code on the page (which I don't control) is converting <b> and </b> tags into &lt;b&gt; and &lt;/b&gt;

Is there a way to alter the above code so it makes these changes? It would be a huge help!