In reply to MG:
We've had a few run-ins with drains at home. It's a long and not very pleasant story and so i shall spare you.
In the first instance, find and open every manhole cover on or near you property and start to work out which loo and drain connects to which. There should be manholes at fairly regular intervals - ours has them at every intersection. Dye is one way of findng out what goes where - the other is coloured loo paper which is a lot easier to acquire. Conduct a few experiments. A soakaway inside a building would be unusual, but perhaps not impossible.
As a start, work out which external manhole lies between your house and the main sewer and work back from that. You should have at least one 'private' manhole. Have a look, and see which way the pipes appear to go and go and find another manhole.
As far as costs go - digging, connection charges possibly. The cost of digging up and repairing a damaged sewer in our road and reinstating the road surface was £8,000 six years ago.
Video surveys cost about £700 IIRC