As an update, I have found a solution myself which I have posted below if anyone else has the same issue. However, it overrides my style sheets, and relies on the WebDataTree element structure remaining consistent, so if the Infragistics staff or anyone else can some up with a more elegant solution I would appreciate it.

My solution was to add the following script, the treeViewItemClick function being called from the WebDataTree item click event;

function treeViewItemClick(sender, args) {
  var selectedNode = args.getNode();
  if (selectedNode != null) {
    var nodes = sender.getNodes();
    for (var i = 0; i < nodes.get_length(); i++) {
      var node = nodes.getNode(i);
      clearNodeBackground(node, selectedNode);
    }
  }
}

function clearNodeBackground(node, selectedNode) {
  if (node != null) {
    if (selectedNode.get_valueString() != node.get_valueString()) {
      node.get_element().childNodes[2].style.backgroundColor = "White";
    } else {
      node.get_element().childNodes[2].style.backgroundColor = "#FFE474";
    }
    // Now check if it has any children, do the same for each if so
    if (node.hasChildren()) {
      for (var i = 0; i < node.get_childrenCount(); i++) {
        clearNodeBackground(node.get_childNode(i), selectedNode);
      }
    }
  }
}