You’ll need an adjustable spanner, a bucket, some plumber’s tape, and a towel you’re willing to sacrifice. Oh, and a new stopcock—buy one with a smooth, quarter-turn handle. The old twisty-turny ones are from the Dark Ages and will fail right when you’re hosting a dinner party. Also, grab a headlamp. You’ll be contorted under a sink like a yoga pretzel, and darkness is the enemy of dexterity.
Pro tip: If you can’t find your tools, a butter knife and a pair of pliers from your ex’s toolbox will not work. Spend the ten quid. Your future dry self will thank you.