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Hi, you can terminate a vent above the level of the highest appliance waste,ie sink. As long as it is a metre away from air vents and Windows it can be terminated anywhere above your highest soil connection. If you have room internally then you can run a small section of 110mm up above the sink then add a 110mm air admittance valve. This does the same as a vent(allowing air in so the fluids move away quickly) but only allow air in and not out to prevent soil smells coming back into the room.you could get away with 3 inch vent pipe with 3 inch air admittance valve.but I would try to keep it 110mm(4 inch) to ensure the best running of fluids away.
The vent can be put between the toilet and shower , but I wouldn't run it off the shower waste ,you might be forever unblocking the loo,
No, I meant don't take the vent off that 40mm waste pipe. Its fine to run the shower and sinks etc into the 110mm pipe, but try to take the vent off the 110mm pipe as close to the toilet connection as possible.
Your welcome. All the best