Coming back to, ahem, square the circle and run the unicorn up the flagpole…
I bought a 2TB Samsung Pro 990 something or other SSD and a nice little house thing for it to temporarily live in. I used Samsung’s Magician Software to clone my existing drive – once I’d discovered I had to remove Bitlocker and reboot to really really remove Bitlocker…
Apart from that, I’m £170 poorer and 2TB richer with almost no aggro. Getting the back of the case off was the most aggravating bit now they don’t do screw on access panels anymore.
Thanks for the advice, pointers etc. Really helpful.