Early to mid 60's, like, say, '63 - '66.
It was made by Raleigh, so the serial number should follow the Raleigh pattern, which you can look up at
The Headbadge.
+1 I will admit that though I'm not usually swayed by anything that has a derailleur installed, I like the color they used on some of the Hercules of this era. Early on after the Raleigh/TI marriage, they were still building the Hercules as if they were a first tier bicycle, as opposed to later when it was clear that Hercules had been relegated to second tier status to Raleigh.