It's worth noting how this may have been handled differently under the Investigatory Powers Bill.
Firstly, there is an actual criminal offence that would have been broken (s8 of the bill). That could result in fines and jail terms. Even if a criminal prosecution wasn't followed, the individuals who broke the rules could have been fined 50k (s6 of the bill).
Secondly, under s171(4), there may not have been a referral to the IPT, as a breach of the ECHR is not itself sufficient for an error to be a "serious" error. And the existence of the error would also only be made public if a "serious error" occurred and it is in the public interest to release the info.
What the IPBill giveth, the IPBill taketh away...