Bathroom mixer shower hot tap won't turn on/turns a little but no hot water and sleeve for changing from shower to bath is tuck on shower. Taps in basin are fine.

What make / model of tap?

Conventional tap or quarter turn ceramic cartridge?

Is this a dedicated shower valve, or a bath tap with shower, or a three way shower valve that can also function as a bath filler?

Hard water area?

How old is it approx?

High or low pressure water system?

What were you expecting it to be made from?

That's not a very helpful photograph.

I had some like that - they were supposed to be held on by steel spring clips but everything had corroded together over time. I eventually levered them off by jamming a large spanner under the edge of the knob and levering it against the top of the spout. A large hammer may have been involved.