okay,isolate water a cold mains near immersion tank.this pipe felt is mega cold.or alternative there shud be isolation valve under the basin.using flat screwdriver turn off until no water drips from hot tap.
remove screw centre of knob.lift and remove.using a wrench and elbow grease unscrew brass nut.remove tap insert.change 1/4 inch washer.replace tap in reverse order.dont rush.
diy expert,heating engineer
December 2012