So using illegally obtained information as evidence
Privileged material will not be admissible in court unless it was created in the course of a criminal act or to further a criminal enterprise.
It is a criminal offence to intercept communication between individuals, such as emails and telephone calls, unless you have their permission (Regulation of Investigatory Powers Act 2000).
In the course of civil litigation you are required to provide your opponent with any material in your or your client’s possession which (among other things) helps the other side’s case or harms your client’s case. If you have acquired covertly or illegally obtained evidence, you may therefore be forced to provide this to the other side (even if you decided that it was harmful to your case and you did not want to deploy it).
However if the information was obtained and published by a third party, Der Spiegel then
"Since the WikiLeaks scandal, the legal parameters for admissible evidence seem poised to change: evidence that would have been considered inadmissible due to its privileged or confidential character is now admissible because it is considered to be public information. Nevertheless, this boundary should be carefully policed, due to the fact that this evidence was unlawfully obtained at some point. Therefore, in a prima facie analysis, the fact of the evidence having been obtained illegally would weigh against admissibility in light of on public policy grounds. Under the reasoning in Caratube, what would happen if one of the parties hacks the other parties’ emails and then asks a third entity which is not part of the dispute to publish this information in order to gain publicity for the purpose of using it in an arbitration procedure (based on the argument public availability destroys the privileged or confidential status of information)? In light of the foregoing concern, evidence that was unlawfully obtained and becomes public should only be accepted by an international arbitral tribunal on the consent of both parties. This will prevent any party from trying to unlawfully obtain information and will maintain fairness and equality among the parties in the process"
Clearly City's lawyers would have a strong argument against the "evidence" from Der Spiegel being used so its doubtful imho that UEFA would risk taking retrospective measures against City