I finally work out a way to do this. If you append #anything to the URL of a page that is already open in Safari, it simply returns to that page without reloading it!
This makes sense although there are a few anomalies. If there is an anchor on the page matching the #anything, it doesn't jump to that point in the page as it would if a link with the anchor was clicked. Also, if there is already a page open exactly matching the URL including #anything, the page is reloaded.
So my solution it so to call the original URL with a #random anchor appended to it.