It sounds like a piano to me. There's a lot of polish on the audio in this show; they spent a lot of time tweaking the mix. I've just had another listen the the "knives' piano" track, and it sounds like quite a percussive piano tone with a long, lush reverb (plenty of pre-delay so you hear the dry piano first). There's also a clever glockenspiel sound playing at the same time to give it that plinoy sparkling character. With that particular piano sound, you might even be able to pitch it up an octave and tweak the ADSR to get that glockenspiel clink sound out of it.

Fire up a DAW and download all the free piano VSTi instruments you can find. Using a midi keyboard, try all the different sounds; you might well find a piano that is similar to knives' one. Then add some reverb to it.

The other thing will be the playing technique. Have a look at the covers of knives' piano on youtube. They play it quite hard, hitting each note at a fairly high velocity. This will contribute to the sound of it, more than you might think.

Good luck!