Heritage shower query

[dean8486]

Hi there, I have been trying to access the cartridge on a Heritage, Hartlebury 1924 thermostatic shower and have ran into a problem. I have removed both the temperature control and the flow handle and now need to remove part of the body to access the retaining nut for the cartridge. On the bottom there is a hex which I presume is for a grub screw. There is one on the fitting manual. The problem I'm having is when I use an allen key to remove it I can't find one that's the right size. They are either too big or too small. I've tried this with both imperial and metric allen keys. Is anyone familiar with this model or know how to solve the issue?

I will attach some pictures

Thanks,
Dean