1. Sometimes, particularly on cheap valves, the plastic lever comes so close to the nylon nut at the end of the washing machine hose that the nut cannot be done up tightly enough, and so it leaks.
2. You could measure the width across the flats of the valve stem (the brass bit) and buy a cheap open-ended spanner of the correct size. Tie a bit of string to the spanner and the pipework and it is there next time you need it.