The current site for EPM would be totally unsuitable for a Radio Range so it must have been on a different site at that time especially as the current building would not have been big enough to accommodate the range transmitters.
I have no knowledge of the Radio Range at Epsom, it predates me by many years, but at Dunsfold the Radio Range was converted into an NDB by retaining the original building and using the Range Mast nearest the building for the aerial. That arrangement continued until the early 80s when the remaining mast was condemned and replaced with the type of aerial now seen at EPM. The Dunsfold Range Building consisted of an equipment room next to a generator room with a firewall between the two which looks remarkably similar to the outline of the foundations in that field.
Originally at its current site EPM would have used a wire antenna strung between two fairly tall masts/poles either end of the compound. There aren't many en-route NDBs left but the building at EPM is at least brick built. Recent Google Map pictures of Westcott show something more akin to a horsebox.