Jim. If the valve is seized then as the other guys have stated first call should be to the waterboard, any repair/replacement will require a shutdown of the supply and probably mean shutting down of several properties..something the waterboard need to do. Something you may want to bear in mind is going on a meter, they can install it where the current valve is and if in the worse case they will not repair your valve then having a meter fitted will save you money...meters have plastic valves that rarely seize and are maintenance free. And the average metered house pays around £1 per day for water. Robert.