Franke sink plumbing kit, sewage smell

[Oliver8]

I've had a Franke 1.5 sink fitted recently using their siphon 2 plumbing kit as part of a new kitchen installation. However, I now regularly get sewage smells coming into my kitchen.

I was wondering whether the air vent has been installed in the correct position. Should it be installed in the lower position which connects to the main bowl rather than to the .5 bowl.

Could this be causing the smells? Or should I be looking elsewhere?

Thanks for any help and picture attached for
reference.

 

[WC1]

With what looks like a big blob of sillycon on the sink waste, pipework across the sockets and tape on compression threads it is typical kitchen fitter fare.
Does the smell come from the vent? - get your nose in there!