After nearly 2 years in the planning, and endless delays / lack of inertia to break SSL/TLS or the MTA or the myriad of other things that I had spent ages trying to make work, I have finally managed to migrate the majority of the content from the old black box to a newer black box, replete with SSD’s and faster CPU.
There will still probably be a few broken links for the next week or two before I can say its finally complete, but at least I can now turn off one of the boxes and not shuffle between the two.