can a vent stack be left inside a building and simply cut down
Say what now..... connected to other drains ... opens in your conservatory....

What are they thinking...

You will be subject to all kinds of gasses, and SMELLS, and possible the odd rodent returning up the pipe. 

Even if there is some kind of trap down there that prevents backflow, the pipe will suck air from your conservatory and make noise.

Tell them to move it.