Domestic gully trap missing a cap or bung?

[objectivedynamics]

We've been noticing unpleasant odours coming from our outside rainwater drains. The toilets never back up when flushed and neither do the rainwater drains. We are never aware of these smells inside the house either. Just outside on the patio near the drains.

I've been told that the "gully pot" should have a bung and that the lack of one might be a cause of the unpleasant smells. I think that the attached photo shows that ours is missing one - please correct me if I'm wrong.

I'm happy to get a professional in to look at fixing this but I'm just trying to get some background so that I stand a chance of understanding what they are saying.
Can anyone tell me if the above seems feasible and if so, whether or not these "bungs" are standard parts that can be purchased from a plumbing supplies place?
Cheers,
Mike

 

[jj4091]

Not a plumber but that looks like a bottle trap that needs cleaning out. A pic from further away might help.

 

[Nige F]

Not sure about a spare plug for that but - a downpipe leaf guard might fit , yes it's full of holes, but they could be sealed with expanding foam or a high spec silicone sealer .- worth a try before you get a tradesperson in. !