Shower Head Leaking

I know very little about plumbing.

The shower is leaking from the head despite being fully turned off at the wall. I also have a problem with the hot water going on/off or not remaining consistent throughout the shower being on, but I don't have that problem if I just run the sink hot tap for example, only the shower has this issue.

I thought maybe it's the shower valve/mixer thing?

I got the faceplate off and the top tap controls the water flow and the bottom controls the temperature.



I figured out how to get the bottom cartridge (?) out but I can't figure out how the top one comes out or if it's even supposed to come out. On the bottom one there was a large metal outer covering that just screwed off and then that smaller nut inside that needs to be unscrewed, but the top one that silver cover just won't budge and its very difficult to get a grip on.

Couple of questions:
A) Is that top tap for the water flow supposed to come out like the heat control valve at the bottom and if so any ideas how because it's not obvious?
B) The hot water valve (seen below) I can't see a like for like replacement on Amazon for example. Is the brass bit supposed to detach from the bottom bit with the spring at the end? Do you just replace the brass bit?

 

Thanks, yeah just found that