Soil stack in the corner of the kitchen.

Photos - https://photos.app.goo.gl/X4P2yaHjSyBThzat7

The rubber washer in the boss has been crushed meaning I can no longer make a good seal. I've tried a couple of other washers but neither are giving a good seal (the usual soil pipe smells telling me it isn't air tight). Does anybody recognise the make so I can try and source a replacement washer?

My only other alternative would be to add a boss which would reduce the amount of height to play with for sink wastes etc.