Any idea how I get my boiling water tap back on?!

[morethanacondiment]

The handle (bit you turn, not the bit where water comes out) has come off our BWT.
We bought it from Appliances Direct less than a year ago, and they say it's user error so not covered. They didn't get back to me when I asked how turning a tap could be user error
I can push it back on to make it work, but can't work out how to make it stay on.
Any ideas?

Is there a screw that goes into that spindle?

Just the little post that's in the middle of the tap, and I can't work out how to screw it in... it doesn't bite... or do you think there might be another bit?

The spindle and the cap part look like they just push together but it looks like it'll be a fiddle to get the teeth to line up.

Ah... happy to keep going if it's just a case of lining it up right. Will go for the element of surprise!