In my opinion it's obviously not ideal to have an internal gully located in the property so I would look at carrying out a small excavation to remove the gully, replace it with a standard 90 degree rest bend to ground level and install either a rubber or plastic 4" to 40mm reducer before plumbing in the x2 waste pipes so that the entire drain is enclosed.
This will definitely stop smells from escaping plus remove the possibility that the gully trap could become blocked and potentially flood your conservatory.
Please let me know if this is something you would be interested in and I could possibly point you in the right direction (in terms of a contractor to carry out the works).
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