Thanks richUK - job done!
Had tried every which-way to withdraw this bleedin cable from its socket, waggling while levering, see-saw fashion either at both sides or one side, pulling, levering - while slightly rotating! You name it, I tried it…!
I was not quite so concerned about the strength of the main box moulding but more the socket and sprung flap unit itself which felt like it might fail at any moment!
Having cut away most of the bulky ‘finger purchase’ area of the cable I managed to get some side cutters close-up alongside the flap - lo and behold as I squeezed into the hard plastic to the side and very close to the flap, out it came?
This ‘old’ cable had never ever felt as though it had ‘clicked’ in positively and looking at the small detents I can only assume it might’ve gone past a point of ‘no normal return’ in some way?
Anyway, not a mark on the WD, I have ordered a new 1m cable on Amazon, £3.95 incl with free delivery; but think it time to consider replacing my (by now quite elderly) Yamaha amp with a newer HDMI model…