Council Tax is a property tax. That means that the amount due is based upon the property (and the characteristics of the people living in that property). Once that tax has been worked out, we then need to work out how is responsible for paying it.
No Council Tax liability exists if every occupant is a full-time student. As soon as that is not the case, then a Council Tax liability occurs - although a discount may apply. In your case, a 25% applies, because the tenant who is no longer a student is deemed to be living alone (as students are "disregarded" when they count how many people are in the property). Therefore the Council Tax liability is 75% of the "full" Council Tax.
Then we come to who is responsible for paying it. There is a hierarchy involved in this (see the "Who has to pay council tax" section of
this page). In your scenario, that would be "a residential tenant" (each of whom would be liable if there is more than one). The key question, it seems, is whether all tenants are responsible for that 75% Council Tax - or are "disregarded tenants" not included?
For that we have to turn to the legislation itself (
Local Government Finance Act 1992, as amended). They key part is schedule 6, "Persons liable to pay council tax", and in particular subsections (3) and (4). (The reference to subsection (2) basically says that a person who is "resident of the dwelling" is liable".)
"(3) Where, in relation to any chargeable dwelling and any day,
two or more persons fall within the first paragraph of subsection (2) above to apply, they shall each be
jointly and severally liable to pay the council tax in respect of the dwelling and that day.
(4) Subsection (3) above
shall not apply as respects any day on which one or more of the persons there mentioned fall to be disregarded for the purposes of discount by virtue of paragraph 2 (severely mentally impaired) or 4 (students etc.) of Schedule 1 to this Act and one or more of them do not; and liability to pay the council tax in respect of the dwelling and that day shall be determined as follows—
(a)
if only one of those persons does not fall to be so disregarded, he shall be solely liable;
(b) if two or more of those persons do not fall to be so disregarded, they shall each be jointly and severally liable."
So paragraph (3) says that each "resident of the dwelling" is "jointly and severally liable to pay the council tax";
however, paragraph (4) (a) says that
if only one person is not disregarded (i.e. is not a student in your scenario) then they are solely liable for the council tax.
So Citizens Advice are right, and the Bristol council tax office are wrong. You are not liable at all, as you are a disregarded person. The only person responsible is the non-student.
If the council tax office dispute this, ask them to read subsection 4 of schedule 6 of the Local Government Finance Act 1992 which specifically says that if only one person is not disregarded ("does not fall to be so disregarded") then they are solely liable.