The pipe to the cold water tap in the downstairs toilet has recently developed a slow leak (one drip every few seconds). How difficult is this going to be to fix myself? I know where the stopcock is, I can take a bike (or a computer) apart and reassemble it correctly, but I have never plumbed before and I overthink drilling holes in walls ("but how do I know there isn't a radiator pipe behind this?") to the extent that I never even put shelves up. What would a plumber charge?