Wiggal's answer refers to a telephone call. It's a legal requirement that anyone recording a telephone call must notify the caller. (That's why you see and hear statements such as 'calls may be recorded for training purposes').
However, there's no such legislation regarding recording normal conversations. I used to be a union rep. I always kept a tape recorder in my pocket when representing members. It came in useful on several occasions when employers later produced 'minutes' of a meeting which didn't accurately record what was said.
I've also worked in journalism and recorded many conversations, both with and without permission. I still carry a tape recorder with me and I frequently use it to record conversations. (If you chat to me in a pub, there's a very good chance that I'll record the conversation, without your knowledge. Also, I often record conversations with shop employees, again without their knowledge, so that I've got evidence of their product claims).
It's perfectly lawful for an employer to record conversations with an employee. It might actually be in the employee's interests that an accurate record is kept of what is said.
Chris