When you read the next paragraph, I need you to take a deep breath and remember that while I may be advanced in years, I have only been into air rifles for 3 months.
It seems what I have is a muzzle brake. While it does not show anything on the Czech website photograph of the HW100T, if you go on the HW100S you can clearly see the grubscrew on one of the photos.
It seems this is the standard in some countries.
Now I have tightened the barrel and used Air Arms Diabolo Field pellets I can shoot 8mm ctc 5 shot groups at 30m. The limitation being my ability, not the rifle.
So I am going to get a 16mm moderator adaptor and either the Weihrauch or Hogan Decimeter silencer.
One last question. The barrel is 15.85mm diameter. Will a standard 16mm adapter be ok, or will I have to shim it out?