What is this?

[Dilbertian]

I found it at the back of the cupboard under the bathroom basin.

The larger bit is hard plastic, flat, with one small flat end, and one larger round end. The round end has shaped lugs that fit into the round cap. The part on my fingers is the softer round cap that fits over the lugs on the hard piece.

My first thought was radiator key, but it doesn't fit any of our radiators.

Meter cupboard key?

I think @TodgerStrunk is right. It’s some sort of utility cupboard key.

I got a similar looking thing with a toilet seat to tighten the bolts from underneath.

I was thinking toilet seat key too!

Not the utilities cupboard key , ours is completely different and I've checked that it's the right key.

It didn't fit any of our toilet seats, either.

The mystery deepens... I've found another:

It's a key for an aerator tap. Google Neoperle.

Ah ha! Thank you. I shall label the blue one forthwith.

What about the black one?

Sorry, no idea. Any words on the other side?

Possibly a special tool for tightening up your bathroom taps, some designs come with their own tools for fitting.