How can I fix hot tap in bath. Boiler is a Compact HE?

The hot tap for the bath is running slow and goes hot and cold...I believe it may be the tap itself (The unit inside it?)
How do I turn off hot water, there seems to be a black stop cock under the boiler to the right. To the left of it is a yellow tap? so that I can replace the faulty tap....if thats what it is!!
Thanks