Soil vent smells

We have been getting sewer smells in both our main bathroom and ensuite toilet. We are in a rented property and reported it to the agent who them informed the landlady who sent her partner round to investigate(he isn't qualified) On inspection the sewer vent pipe goes up through the house (an extension was built on the house) ending on the loft space of the extension and according to the landlady's partner appears to be capped (he looked from the hatch). His answer to solving the issue is to add wood end plates to the already fitted boxes that hide soil pipes coming from both toilets. I'm concerned that this isn't the solution and where will the smells be forced to come out of next - the loft hatch? I know that the air vent pipe can end in the loft as long as it has a certain valve attached and that there is an additional vent going outside. I'm not 100% sure whether there is the additional vent but there is a random vent on the wall of the ensuite toilet which I've deduced that might be the vent on question. All of this info I've sourced myself so really not very sure if it's all correct, but I'm wondering if all is fitted legally I'm the loft could there be a crack in the pipe? Is it absolutely coming from there? Can sewer smells still come through the sink traps despite water being in them? Thanks for your time Jo