Hi Adrian,
just had my MOT in the Netherlands on my Pantera and parking brake was my main worry as I was missing the return spring on my LH caliper. I found that you can adjust the hand brake quite well by turning the 17mm nut but you need the return spring (see pics) to have the piston release properly after releasing the hand brake. Luckily I got a break from the inspector and got my approval.
One tip, ensure that your new brake cable is not touching the hot exhausts. On my exhaust, previous owner welded a plate for that purpose, see pic.
Gr.
Joep