Stuck mixer tap on the bath

[PCAnimal]

Hi I wonder if someone could give me some advice please?

I have this mixer tap on my bathroom which is almost impossible to lift up now. The center used to lift with ease but now it feels welded down. I unscrewed it and found a piece of broken rubber which looked like a seal of sorts. I can't lift it up much and only get a dribble out of the head.

I assume I need to add a new ring or something similar? Can anyone advise please?

 

[foxhole]

Sounds like the O ring has failed and maybe obstructing the pin, needs replacing.

 

[PCAnimal]

I can get those from screwfix. Do they come in a standard size? I assume I unscrew it and slide over the top? I've not replaced one before