ChrisM, the water hammer arrestors in the US that I am familiar with are a completely sealed assembly, with no means to adjust the internal pressure. Many on them are rated for inaccessible installation, meaning that there is no means of servicing them short of removing wall or ceiling coverings, so a charge valve such as you mention would be a bad idea.
What is the normal range of water pressure that you see in the UK? I don't know if plumbing pressure is figured in Bar of KPA. I am somewhat familiar with Bar pressure conversion to our pressure measurement pounds per sqaure inch (PSI). Normal pressures here range from 40 PSI (~9 Bar) to 80 PSI (~18 Bar). Above that, code requires a pressure reducing valve to moderate the pressure before it enters the dwelling. The hammer arrestors that I have examined are usually precharged somewhere in the 50 PSI range, so they service the range of pressures that we experience relatively well, in my experience.
Thanks so much for participating in the forum, I appreciate the different perspective that you bring.
Cheers!