That would seem logical, so logical in fact that, although it was pretty easy to see that it didn't fit, surely I must have missed something while fiddling with it in the dark the other night.
Just read the description of the camera on this site:
https://www.astrofactors.com/compone...l-ii-mono.html
and it says it includes a "1.25 extension tube / filter holder"
So I'm starting to think the included IR filter has a non standard thread size 
Worst case it's nothing a bit of electrical tape can't fix
I'll have a look when I get home.
EDIT: the description says the IR filter is 22mm. Is that standard?