Hi Dean, I had a 3kw element with armoured cable long enough to reach my shed from the van . This had a normal 240v plug on the end which I plugged in each night for hot water in my tank.
It was same setup as you.
I know this is dangerous because the plug can overheat and eventually set on fire.
But my point is, I did this for quite a few years and the system worked well. So, if your elements went pop, it seems likely that they were faulty or/ also maybe your wiring may be dodgy.
Did you get any hot water in the tank , or was it stone cold the next morning?