Thanks for your comments
All the 2 bedroom apartments in the development (over 450) are the same size within a few square metre and the IBI bills for 2 bedroom seem to range between 430 and 450 euros so my demand at 716 seems to definitly be a mistake. Even the 3 bedroom duplex apartments are less than this.
I have emailed the Council and await their response as it looks like they have been overcharging me for the past 2 years as well.
Do you think I should dispute the current charge and just pay the same as other 2 bedroom apartments and notify the Council that I will only pay the balance if they can produce a valid reason for the difference.