Apologies for delay in replying.
I am sorry to trouble you, but have you any further information of meeting on September 23?
Sorry to bother you so soon again. Is airport of destination going to be Heathrow or Gatwick?
Sorry to chase you, ------.
I'm sorry about what happened yesterday.
It won't happen agan.
Our apologies for any inconvenience.
I apologize again for the delay.
I apologize for the short notice.
We are very sorry about that.
We would appreciate your understanding.