I feel you should discuss this with a solicitor, particularly in view of my first answer about sewage overflowing from the rodding chamber. If you do decide to allow this to go ahead, you really need to negotiate a substantial amount of compensation. I mentioned discussing it with a solicitor, as you would need a water-tight agreement drawn up, stating that the party installing the sewer drain would be liable for the cost of cleaning up any overflow (believe me it happens), and also including the cost of restoring your garden to its original state. Please think very carefully before agreeing to anything.