Replacement of bathtub mixer tap cartridge

I have come across some cheaper mixer taps that do not allow access to the cartridges so they had to be replaced but most branded taps will have an instruction video online explaining the process, hope that helps 👍

If your mixer has a leaver handle this sometimes can be unscrewed which allows access to the grub screws Best of luck jim

Firstly I would remove the trap it may have fallen into bath and could still be in the trap if not then contact the manufacturers of tap and order a new grub screw is the cheapest option if the taps are faulty then just replace the taps