Smell from recently capped chimney

We had our chimney capped a couple of months ago (the next-door semi were having theirs done, so we said “yes” to their contractor doing ours at the same time). The contractor has removed the chimney pots and all that is visible from the ground is sloped mortar on top of the stack. It’s possible there is no ventilation at the top – there’s certainly nothing protuding very high above the top of the stack and no air bricks are visible.

The fireplace is still open (just covered with a sheet of plywood with a hole in it) in the room below.

A couple of days ago a very strong dusty/sooty/musty smell started in the room with the fireplace. There is also a noticeable draught coming from the fireplace. Neither of these were present at any time before the chimney was capped. This has coincided with a period of strong winds (though I can’t see any damage to the chimney stack).

Is it possible the smell is due to condensation from the now inadequately ventilated chimney, and the draught is just due to air pressure changes in the room (annoyingly it’s worse with the windows open) sucking air in and out of the closed chimney? Or is it more likely the stack has been damaged nd leaking?

And am I right in thinking the correct action is to get the chimney properly ventilated at the top? Is it going to need cleaning as well?