You need to calculate the entire tax for the year, divide by 365 (number of days in year) then multiply by number of days resident at property. Then, subtract any payments made, and add on any costs, and that's what you owe.
You have lost the right to pay by instalments, however, you can call your CTAX office and make an arrangement to pay over a period of time. They may, depending where you are in the recovery process, take you to court, but not act on the Liability Order unless you default on the arrangement, in which case they can attach to your earnings or benefits, or refer to the bailiffs.
Talk to them, as normally they are more than happy to make arrangements as it costs more to attach etc. They may ask you to complete a form of incomings/outgoings though, unless you make a sensible arrangement with them. If it gets to attachment, they will just take it out of salary without looking at outgoings, so I'd get it sorted soonest.
If you still own the property then if it is unfurnished you can ask for an exemption for 6 months, otherwise a 50% discount if furnished. If you rent, unless the l/l gets someone else in straight away, you will remain liable until the end of the tenancy agreement. Same rules apply to the exemption/discount though.
Hope that helps. I used to be a scary Council Tax Officer, before I became a Scary Teacher, and we don't bite honest...only if someone has nicked the chocolate!