How do I bathroom / kitchen / extension? etc.

Moved house in march this year. There's always been a smell from inside the wall cavities in the bathroom. No extractor fan, so it's been necessary to keep the window open so that the room doesn't get too stuffy. The smell is unpleasant, but not horrific.

Medium to long term we'll gut the room and have the whole bathroom re-fitted. Short term, we can't immediately fit a fan because we are in a conservation area, so would need planning permission or dispensation (it's in the works).

Recent cold weather meant we had to shut the window; the smell got annoying again so I investigated.

First problem: the basin waste outlet is push fitted into a 90° elbow that disappears in the wall cavity. The elbow is legit push fit, but the trap outlet isn't. The fitters have apparently compressed the joint to get it to hold. In so doing they've made it egg shaped, and pulled the o-ring into the gap. So the basin has been leaking into the cavity.

Second, the toilet waste is plastic and connects to the old cast iron stub pop-up. Access is really difficult, but today I managed to get a photo which seems to show the plastic pipe is just slipped into the cast iron, rather than use a proper fitting.

Any thoughts on how to solve these in the short term? I'm thinking caulk the joints with plumbers mait - not ideal, but if it stops the smell then I could look at a more substantial solution. The pipes are all buried in the walls, along with a shower bilge pump (so I really don't want to be cutting holes for access, since it will expose an electric element to the splash zone).

Properly annoys me to see shoddy work like this. The bathroom fit was done for the previous occupant who had mobility issues, and it cost £12k. The room is 2.25 square metres.

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