Whether the recording is illegal depends on what the contents of the conversation were. The Data Protection Act will only apply in situations involving processing of personal information. If you have recorded anything that involves sensitive personal information about other people then you may have to act within DPA principles. But if the information is to be used for litigation purposes' then there is an exemption from the DPA and it would not apply.
I would however bring your attention to the case of Dogherty v Chairman and Governors of Amwell School. Mrs Dogherty made a claim for unfair dismissal, and, in the course of the case, it emerged that she had made a secret recording of both the "open" disciplinary meeting and the "private" appeal meeting. Her contract of employment did not expressly forbid covert recordings of meetings.
She won her case, but the school appealed on the grounds that the secret recording infringed the human rights of the school governors present at the meeting.
The EAT rejected the appeal, finding that the central issue, as least as far as the "open" meeting was concerned, was whether the content of the tape was relevant to Mrs Dogherty's claim. It was, so this section of the tape was allowed as evidence.
However, the "private" section of the recording was ruled to be inadmissible as evidence. As this portion of the tape was a full and frank exchange of views between the governors themselves and was conducted in private, it was not directly relevant to the way in which Mrs Dogherty had been treated in the way that the open meeting had been.
As this is an EAT decision, the tribunal would be bound by it so it is worth thinking about whether your particular situation would be covered by the above.
I hope this helps.