Your water board will have a list of underground contractors who will be able to help. You might want to look at something like household insurence to help you make some sort of claim against the guys who left it like this state, scandalous, IMO. If someone made an honest mistake I'd usually say get them back and give them a chance to put it right, but not in this case. I'd keep lifting the lid every few days because you won't know if it's blocked because the waste will just leak out by the gas pipe. (which is one reason why you can't just cut into an I/C) - and it will block again because of the soil falling in at the breach. Look on the bright side, you found it, and you can put it right rather than building contol, the water board or some other body finding it - that could have caused you serious issues. You've come to the right place to get some input and you should be able to recover the cost from the other side. Merry Xmas to All and best wishes for the new year, WC1