Maybe a better idea is to replace the existing sawn off connector on the stack and get one of these instead?
https://www.ebay.co.uk/itm/4-Soil-P...0-110-110-90-87-SEWER-WASTE-VENT/111911500778
https://www.ebay.co.uk/itm/4-Soil-P...0-110-110-90-87-SEWER-WASTE-VENT/111911500778
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