If this is any help......
The same thing happened to me in the last house I lived in. It was a 1930's semi and there were 4 houses that fed into the same sewer. Our next door but one neighbour knocked and said he was having problems with the manhole in his garden, sewage was seeping form under it and down his garden. He told the 4 houses concerned that he was notifying the council and water board, but that the water board had told him that the sewer would have to be dug up and that all 4 houses would have to pay an equal share. The work was done and the water company billed us for the work - apparently part of the pipework went up a slight incline which contributed to the problem!
In that case, if any of the householders hadn't paid for the work, the water board would have taken action to recover the money.
When the pipework was unblocked prior to the work being done, it was found to be blocked with disposable nappies - the neighbour who was having the problem had caused it himself, his wife had been looking after their new grandchild for a few days and apparently didn't realise that you couldn't flush 'disposable' nappies away down the toilet!
