Hi mate, yes i work for Southern, the recruitment process length vary depending on the role and the current circumstances in terms of demand for roles to be filled.
But I would say yes the process is long, very long actually compared to other industry sectors. On my induction everyone said the same thing, that they applied ages ago and had almost forgotten about applying by the time they were called in to do the tests and then less of a delay before being called for an interview, it's the initial screening process that seems to take the time.
I would say the lesson here is to just relax and don't stress out about it, you will hear when you hear, you could always drop HR an email to show how keen you are, can't hurt right.
Hope that helps mate, cheers