Blocking up first floor fireplace

I want to remove the fireplace in a first floor bedroom in a Victorian mid-terrace house, and seal it up.

I've removed the fireplace-surround and hearth, and have got the bricks (including air brick) to seal up the opening.

However, it occurred to me that it's not immediately obvious what the opening of the fireplace is built upon (and whether it can take the weight of the 50 or so bricks that I was planning to lay to seal the opening).

Has anyone ever considered a similar situation?

The dining room below also has a fire place, so I presume that this fire place in the first floor bedroom will be built on top of that (and therefore able to take a decent amount of weight). I will also be using frame cramps fixed into the existing brick structure.

Nonetheless, it's not definitively obvious that that is the case - so any thoughts or experience greatly appreciated! It would be nice to avoid 50 bricks and 50kg of mortar crashing through the ceiling onto the dining room table.....

Posting a pic of the bedroom fireplace showing where you have removed the hearth will help but, typically, its fine & safe to block up a bedroom fireplace opening with bricks or blocks or a bit of framing and plasterboard.
Have the flue swept before blocking up and venting.

Thanks very much for responding Tel. Here are some pictures. The old hearth was under what is now covered by the piece of plyboard.

There was an old piece of concrete in the fireplace itself which I took out at the same time I took out the hearth. Underneath that were a few broken up/uneven bricks which I also took out to lay the more level strip of mortar you can see in preparation for the bricks to seal up the opening.

Yes, the chimney has already been swept, and we're going to get it capped. I've got an airbrick for ventilation.

The old piece of concrete in the fireplace was the back hearth, and should have been left undisturbed.
No matter - you can still brick, block or board.

What a shame. And devaluing the house too.

Thanks very much Tel.

Ah well. Good to know for next time!