I've turned my mains stop valve off to replace a dripping tap washer,but it isn't running off even though I've opened all the taps in the house..

but it isn't running off
What does that mean?

If you mean that the water is not stopping, then the main shut off is not working and you will need a plumber to repair this.

Chuck

Shut the water off at the street box. This is probably where the meter is located.