Hi, thanks for the reply.
I have been meaning to update the post.

I emailed Hull Cartridge to ask for the size of the Allen key for adjusting the trigger, that was about a week ago, still waiting for a reply! Meanwhile I searched the web and the only place I could find the information was on the excellent "HW100Tuning" website. The Allen key size is 1.2 /1.3 or 050". I think it's a poor show that one wasn't supplied with the gun.