Secretly recording conversations at work might be against company policy (check your employee handbook) whilst also being a grey area in UK employment law. However, what is clear is that once you have left your employer, there is very little the employer can do to you if you actually made recordings of conversations whilst being there. Whether a tribunal decides to admit the recordings is a chance you have to take. Still, if the recordings prove that the employer is lying about events or statements made, then a tribunal might allow this as it helps to paint a picture of the employer. One important point to keep in mind is that you should only record conversations where you are present - you won't be able to leave your recording device in the room if you leave.