Well ballcock washers anyway. The one in the cold water tank in the loft in me new house is leaking but due to the tangle of roof trusses up there it's a nightmare to reach the tank, crawling under and over some of the horizontals, trying not to step on the ceiling between the joists and generally fumbling around in the dark as there's no light at that end of the loft.
I managed to get to the tank and turn the inlet tap off as a temporary measure to stop the overflow dripping but have to face it again soon to change the washer. I'd rather buy every possible size of washer first than go up there once to pull it apart and then again to fix it after buying a washer. It's a brass unit about 30mm to 35mm in diameter (I'm guessing as I didn't have a measuring stick with me) with the arm sticking out from underneath that.
Is it likely to be one of a very few standard size washers or do I need to take it apart first?