I don't know about pipe size on the extended bit I will have to find out but it is the same size as the soil pipe (I think) - Is it possible this isn't working because the end of the soil pipe still has an AAV (behind walls as stated above) on it that is possibly malfunctioning? or should the extended pipe pull the gases/smells through to the outside . Thanks for all your advice.what size pipe has been used to vent? it needs to be 2" (50mm) minimum. 40mm will NOT suffice.
PLUS you should fit anti vac traps to basins & sinks.
why? because its quite likely that a trap is having its seal sucked out. if you fit anti vacs everywhere you can it helps reduce that impact by allowing more air into the system.