I have added a two bedroom with a bath building, it is a separate building and is at a lower level than a patio net to them. When the air movement is out of the west we get a sewer smell
Would a studor vent work OK on the roof vent to stop the smell

The prevailing wind in the US would blow the "aroma" that way. A Studor Maxi Vant will fit a main stack but as to whether it is approved in your location and for that purpose is up to the local authorities.