Luckily I had a stack of these sort of banana sockets (albeit for old school jacks) that just so happen to be a perfect fit, and after investigating an exact replacement I discovered that they were originally of a distinctly poorer quality than the ones I had in stock, Also luckily my partner had a kids birthday party to attend with our kids so I managed to find a couple of hours to affect a repair (and a pretty nice one at that) Confident that the issue was resolved I went straight back to testing, Successfully now the melted plug has been replaced, I replaced both the –ve and +ve connectors just for ascetics reasons and made a much better mechanical connection that the poor soldered connection that were present originally.
I think the reason for the meltdown was due to the fact that my battery testing rig has a set of buss bars that are connected via banana plugs to the Powerlab 8 which means that I am not plugging and unplugging the banana jacks after each test (as you would normally if you were just testing a single cell pack) so any degradation of the banana jacks was not being noticed until it caused a problem (i.e. melted) So in order to prevent the same happening again I replaced the faulty banana jack plug with a shiny new one (stripped from a dead cell pack) and cleaned the banana jacks pin with wet and dry paper. I will just have to check the jacks more often between cell tests.
I can honestly say that if there is one thing I have learnt (the hard way) it is that the most common cause of problems with HV EV tech is bad electrical connections.