Basically when I debugged the error using IE Developer toolbar, I saw that the error was being thrown within sp.ui.runtime.js file each time I was trying to change the Page Layout - namely "Unable to get property 'showWaitScreenWithNoClose'" and also the SP.UI.ModalDialog box was null.
Eventually I traced the error back to the order of key js files that should be included/loaded on the default master page for the site. In my case the culprit were two script references:
<script
src="../../_layouts/15/SP.Runtime.js"
type="text/javascript">//<![CDATA[
//]]>
</script>
<script
src="../../_layouts/15/SP.js"
type="text/javascript">//<![CDATA[
//]]>
</script>
I changed the above lines in the default master page to:
<!--SPM:<Sharepoint:ScriptLink
runat="server" Name="SP.js" Localizable="false"
ID="s1" LoadAfterUI="true"/>-->
<!--SPM:<Sharepoint:ScriptLink
runat="server" Name="SP.Runtime.js"
Localizable="false" ID="s2"
LoadAfterUI="true"/>-->
The key thing to note is the "LoadAfterUI" attribute that seemed to do the trick,
Now my page layouts are selectable and I can click and change the Page Layout to one the available layout pages in my site collection (Note - ensure that the page is in Edit mode for you to change the Layout Page)