The front garden tap was leaking. I would replace the washer. How hard could that be.
Undo the top of the tap. First problem. The pipe was only loosely fixed to the wall and I had no firm anchor. First solution, use another wrench to hold the tap (against my knee) while I undid the top.
Go to big warehouse place and buy a new tap. Umm. Is it 15mm or 20mm? Didn't bring old tap (it was busy keeping the reservoir from emptying into our front garden). Make educated guess. Buy 20mm tap. Take off old tap. Realise educated guess was wrong. Back to big warehouse (conveniently open lots of hours on the weekend). Buy new tap.
Second solution, replace whole tap. Success!
Sometimes the smallest jobs can take just the most effort.