Loft boiler drain

[Loofah01]

This is the set up my builders left for me. The drain from the boiler goes into the top of a 4" waste and is covered with a rubber 'cap'. The angle of the photo makes it look like the white pipe does a u-turn but it doesn't, just the crap photographer that I am!
The pipes have been attempted to be sealed with silicone which hasn't bonded at all so I'm left with some condensation coming from the 4" pipe, just the occasional drip, plus a bit of a funky smell when close to the pipe.
Hoping that @pigletjohn could give an opinion in particular

The 4" pipe might be the top of the soil pipe, rising from the WC below. They are more often in the corner of a room. Has an additional bathroom been added since the house was built?

It looks like it was done by somebody who has heard of plumbing, but never seen any.

Yes the soil pipe comes from the bathroom below but it's the connections and finishing that are bothering me!

The soil pipe ought to be ventilated. Most often rising through the roof. I wonder if it used to do that before the boiler was added?

When an additional bathroom is added after the house is built, the soil pipe may not be ventilated to avoid effort. Usually they fit a Durgo which is not as good, but better than what you have.

A competent plumber can fit a proper connector with ease.

No this is all brand new, they just did a shite job in some instances. I did wonder about a durgo; this is all in the centre of the house but they could have easily extended the soil pipe upward through the roof. Het ho, guess I'll add it to the extensive snagging list. 7 months and counting...

I've recommended they fit an extension to the soil pipe, connect the boiler waste using a boss connector and a durgo on top. F knows why I'm making the efforts as they should be able to do it themselves.