Opening the old manhole underneath the new extension

Hi, I had an extension built and the old manhole was covered and diverted to the patio, so that I could built over a communal sewer pipe. I had a drain survey done last week and I have been recommended to re-open the old manhole which is in my new kitchen. Because the old manhole had two branches connecting to the communal sewer pipe and there is no way to service the two branches if they are blocked. Is this really a problem serious enough to reopen the old manhole? Thames water, who is the owner of the communal sewer pipe, said they do not allow internal manhole due to risk of flooding and smell even with the double seal.