From what I read, my (M8000) i-spec B shifter should be backwards compatible with my old i-spec A (M675) lever. But I’m not 100% sure.

I think the nut and bolt at the bottom of this pic:

Nuts and bolts

are what I need to do it but the shifter didn’t come with this nut, just the other bits on that pic.

Anyone know whether I’m missing a part that should’ve come with the shifter or do I need to buy this surprisingly priced extra?

B shifter will fit on either brake lever with that kit. A shifter will only fit on an A brake lever.

But isn’t M8000 I-Spec II not I-Spec B?

Edit: Seems M8000 comes in both flavours

An aftermarket M8000 iSpec b shifter should have come with the parts you needed. There should be all the parts that you linked to on CRC… or at least the last couple of dozen I bought did anyway.