Flower - the issue of Duty of Care has to be combined with "reasonableness" which means if it is/was reasonable that you as an ATCO knew something or should know something, and that it is/was reasonable that you could do something, and didn't, you're guilty of a failure of Duty of Care. A Court would ultimately decide on "reasonableness".
So, in the context of recommended vortex wake separation, after having passed the criteria and having had the pilot acknowledge it, if that pilot OBVIOUSLY turns final with less spacing than the criteria (and an ATM can give a very accurate indication), it's reasonable that ATC should repeat the warning and receive an acknowledgement. Thereafter, it's also reasonable that the pilot should make the decision as to whether or not to continue the approach (Duty of Care again...) and unless ATC becomes aware of some additional factor and fails to communicate it (e.g. wind shear report from the landed aircarft), any subsequent accident would probably be put down to poor airmanship.
However, it could be a Court decision if civil litigation for damages was pursued by the pilot or occupant(s) or if deceased, their relatives.
Sadly, adherence to MATS Part 1/MATS Part 2 or JSPs no longer always protects your "six o'clock" (cf. the debate on ATSOCAS) so make sure you've always got a good lawyer available.
If you can't afford one, join GATCO (or Prospect or Unison) and take advantage of their legal representation insurance!!
