I am installing a new bathroom and will be running new drainage to the shared mains drain (terraced house) at the back of the house. I have dug to the shared mains drain, and it is a clay pipe, the circumference is 63cm and approx 20 cm across. I will install an inspection chamber , 110mm underground pipe from my house to the chamber. What size would the clay pipe be? And need to be reduced to enter and exit the inspection chamber? Also would the base of the inspection chamber need to be concerted at it's base?