Have to agree with Darren regards the job Tom.
The company need a profession this one, a company with camera's that can get into the drain system to see the internal damage. I have some experience of ground work and drains from working on building sites many years ago, and its a specialised job. And can be very costly, especially with large commercial drains as this one.
Best advice you can give the company is for them to seek a specialist drain company with all the correct equipment and save yourself a whole lot of trouble and possible cost in trying to do it yourself. If it goes t*ts up, you'll loose you're good reputation with company.
Regards Phil