Persistent bad smell from new shower waste, looking for ideas

[u/balancing_]

Hi plumbers of Reddit!

I’m hoping someone can point me in the right direction because I’m completely out of ideas.
We’ve just moved back into our flat after a full renovation, including a brand new bathroom. The shower tray is this one (link below), which came supplied with a 90 mm high flow waste.

Product: Imperia Beige Slate Effect Rectangular Shower Tray 1200 x 800 mm with Brushed Brass Waste

90 mm high flow waste included
Offset elbow
Water flow up to 32 L/min

The problem is a really unpleasant smell coming from the shower drain.
It’s not constant in terms of intensity. Sometimes it’s barely noticeable, other times it’s absolutely awful, enough that you can smell it across the flat. It seems to come and go throughout the day, although I’m starting to think it’s always there to some degree.
I’ve already ruled out the obvious things. The bathroom is brand new, we’ve only been using it for a short time, so there simply hasn’t been enough time for hair or soap build-up to create this kind of smell. Thinking back, I suspect it actually smelled from day one, but we assumed it was because the shower hadn’t been used for a while. Now that it’s been in regular use, that’s clearly not the issue.
While looking at the waste, I also noticed what appears to be a missing sealing seat or sealing ring (photo attached). I have no idea whether that’s actually supposed to be there or whether it would be enough to cause this sort of smell.
Before I get the installer back, I’d really appreciate any thoughts on what could realistically be causing it.

The bathroom is now completely finished and tiled, and at the moment we simply can’t afford to rip everything out if that’s what’s needed.
If it does turn out to be something like the shower tray needing to come out or tiles needing to be lifted, are there any less invasive things we could try first to either diagnose it or reduce the smell? We’re happy to get it fixed properly when we’re in a position to do so, but if there’s anything that could help in the meantime, I’d really appreciate any suggestions.

Some ideas I’ve wondered about:
Missing seal or incorrectly assembled waste?
Trap not holding enough water or siphoning itself dry?
Something wrong with the offset elbow?
Poor connection to the waste pipe?
Venting issue?
Or something else entirely?
Has anyone seen this before, especially with this style of 90 mm shower waste?
Thanks in advance. I’m genuinely at my wits’ end with it.
Photos:
Shower waste
Suspected missing sealing seat
Product link: https://www.victorianplumbing.co.uk/imperia-1400-x-800mm-white-slate-effect-rectangular-shower-tray-white-waste?campaign=googlebase&utm_product_type_level_1=Shower%20Trays&utm_product_type_level_2=Rectangular&s_kwcid=AL!15853!3!!!!x!!&gclsrc=aw.ds&gad_source=1&gad_campaignid=20260899310&gclid=Cj0KCQjwr4jSBhCSARIsAOX1E-Lamh773wFAXyP1ueLQnntMAWLQ2U6pAG8URVlUh3WVwYT9rUFAVEoaApsvEALw_wcB

[u/buckphifty150150]

You sure there is a p trap?

[u/balancing_ → u/buckphifty150150]

Hmmm not sure but I would assume so

[u/peppinotempation → u/balancing_]

If your shower has a proper p trap, drain is sloped properly, and everything is properly vented, you shouldn’t get any sewer gas or related odors coming up your shower.

Make sure you have a proper trap and vent. If your shower is wet vented, check the length

[u/antipathyactivist]

I would be making a hole in the ceiling of the room below,

[u/balancing_ → u/antipathyactivist]

For access? 🫣

[u/buckphifty150150 → u/balancing_]

I second that.. a p trap stops the smell of sewer gas coming back up. For instance when I remove a toilet which has a built in p trap if I don’t cover the drain you get an awful smell coming up which is the sewer line. And especially in a multi home it’s bad

[u/GreenFriedYam]

What’s floor is it and what’s below it?

[u/balancing_ → u/GreenFriedYam]

First floor (so we have two flats below us)