This will be a private sewer and hence the householders are responsible for it (but I think you acknowledge that much).
To find out whether you are jointly responsible for the whole system, or whether each person is responsible for the bit that runs in their own land, you will have to consult your Land Registry property title. There will be (what is known as) an easement, which entitles you to run the pipe through your neighbours' land, and there will also be a restriction that defines the responsibility for maintenance.
It is usual that householders are jointly responsible. I am preety sure that the council (who kindly sorted the problem out for you all) will have checked who they should be billing (i.e. number 166 or all of you) before they billed.
But there is no harm in checking.
Though hindsight is a wonderful thing, insurance is available against this sort of thing happening - the Water Utilities tout this insurance product every time a bill drops through my door.
Some other general useful in this leaflet, and by the way, the Government has announced that utilities will pick up the responsibility for private sewer maintenance from April 2011 in England & Wales.
http://www.ccwater.org.uk/upload/pdf/private_s ewers.pdf