It is not illegal to record anyone’s voice.
Therefore there is no reason why you should not use the recording.
However they will object to it in which case you can produce the transcript. Also have both to hand.
The rule with regard to illegally obtained evidence changed some years ago and now even if it is blatantly illegally obtained it can be admissible if it’s in the interest of justice to do so.
Let’s say for arguments sake they said something and you have a recording and they now lie through their teeth saying they did not say that.
It would be totally unfair for them to say it was an illegal recording and not admissible.
Go for it.
Can I clarify anything else for you?
Please note that my answer is based strictly on the information that you have given me. If you have not given me all the information, then my answer may be incomplete or wrong.
I am happy to answer any specific points arising from this.
If you still need any points clarifying, I will be happy to reply because the thread does not close.