I stand to be corrected but as I understand it you would be highly unlikely to be breaking any privacy or data protection law.
Section 36 of the Data Protection Act states that:-
Secret recordings are, in many circumstances, accepted in court even if the person being recorded didn't know it.Personal data processed by an individual only for the purposes of that individual’s personal, family or household affairs (including recreational purposes) are exempt from the data protection principles and the provisions of Parts II and III.
Always seek legal advice BEFORE using such evidence and seek proper guidance on how to present it though.
Food for thought