This metal bookcase component called

That's likely a cleanout for your sewer line. When opened, you can run an snake down the line to remove any blockage without ripping out your entire foundation. Here's what it would look like from the side:

And the cap itself can have different styles:

It's called a spanner slotted (or slotted spanner) head. Useful site here. Hard to find in the UK, why the owner used one I cannot imagine. Screwfix doesn't have them. This site may do, although it is international. You'll need to figure out the correct size before you order.

Alternatively get a real cheap flat-head screwdriver the right width and file down the middle until you get the right notch.

If you can grip it with a pliers try a pair of locking pliers and unscrewing it that way I keep a mini vise-grips handy for that sort of thing.