Hello again. I achieved a GB locale format by putting this code at the bottom of another page, defining a paragraph id ("datemod") and adding a paragraph class (p.dm) to the .css document. It shows the date string in the document in GB format but, unfortunately, with a zero time which I don't want.
<script type="text/javascript">
var dateString = document.lastModified.substr(0, document.lastModified.lastIndexOf(" "));
var date = new Date(dateString)
var l = date.toLocaleString("en-GB")
var s = "<i>Document last updated: " + l + "</i>"
document.getElementById("datemod").innerHTML=s;
</script>