My fingers now really hurt from undoing lots of screws, many of which were done up rather too tight.
Only one piece left over: at the very last step I couldn't get part of the mounting bracket off the old hard disk as the screw was completely jammed. Thus the new hard disk is held with blu-tac.
Now trying to install MacOS on it. Hopefully it will work this time.
Edit: Posting from my Mac laptop; it works again!