My sister-in-law's ballcock valve assembly in her hot water cylinder's header tank does not completely shut off and water is coming out of the header tank's overflow pipe.
Am I legally allowed to change the faulty ballcock valve by turning water off; unscrewing the pipe attached to the existing ballcock and replace the ballcock? Or, is the most DIY plumbing work I'm allowed to do is to change tap rubber seal or change taps?

Thanks