Impossible to say without more information, but you can find out the Council Tax and water rates on the property before you put an offer in.
Will you be using a tumble drier, cooking a lot? At home all day, or out most of the day? Power shower? Double glazing and insulation? It will all make a difference.
You will need to budget for:
Council Tax
Water Rates
Buildings and contents insurance
Television licence
Gas
Electric
Telephone / television / broadband
Also service charges are usually attached to flats - make sure you know how much this is going to be and what is included.
I'm sure I've forgotten something in that list. :)