Turn the water off, easier if you have isolating valves. dismantle the tap head, remove the valve, chuck valve in decaler solution (same stuff some use for kettles) Reassemble tap head, turn water back on.
An MCB is an overcurrent protection device employed to protect electrical circuits in a house from dangerous overload conditions should something go wrong.
Get a DIY book it aint difficult. MCB, dunno about that some private joke I think! Oh no, I see, wrong forum section mate !! Have a search on the plumbing and central heating section try search for "replace washer tap" minus the quotes, you should find out what you need.
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