You can get a replacement Stopper if so desired, but it wont be airtight, although can be lightly mortared in if required. https://www.knowlesdrainage.co.uk/product/traditional-drainage-pipes-and-fittings/stopper-plain/
However I still would have reservations with anyone not au fait with Confined Space Working, entering the chamber to fit it.
My view remains, the odour is a separate issue, and needs further investigation.
However I still would have reservations with anyone not au fait with Confined Space Working, entering the chamber to fit it.
My view remains, the odour is a separate issue, and needs further investigation.

