Interview with old unit

I don’t think there’s much you can do at the interview at this point. They know you, your skills and knowledge, what it’s like to work with you. If they liked you before, and you left in good terms, you shouldn’t have much problems.

the only thing they could question is your commitment to them, depending on how long you were there for and how long you have been away. If they think you only going to say short term and move on quickly, they may not want you back. 

You might ask what's changed in your absence, and use their answer to express how much you are looking forward to contributing to the evolution and development of the unit.

Way back I went back to work on the same public hospital Infectious Diseases unit I had left over a year previously, my experience in a swish private hospital for the very rich being not at all what I anticipated. I found the interview was OK in that I was able to justify that the new job was not what I wanted, I had given it over a year and valued the surgical experience etc. I think you need to be able to show that you accept that things change and you don't expect to just step back in where you left off - both you and the unit should have evolved.

Good  luck.