Thanks for the reply guys, I've been in S'pore for a week so excuse the late reply.
Luke, the thread isn't damaged, the cap screws in perfectly without the o-ring. For the time being I've packed it with grease. I didn't receive any instructions with these handles.
I suspect they've shipped with the wrong o-rings (thanks for checking Joseph), the tolerances between the cap and carbon fibre are way too tight to fit the bulky o-ring that came with both handles I bought.
Now I just need to find somewhere to get some thinner o-rings, tried Bunnings but couldn't find any thin enough.