how can I free a seized stop tap in my loft?

read all of this first and ponder what to do !!!!!!
The tap has probably seized onto the spindle gland easiest way i reckon is this.
first( isolate the incoming water supply) as it enters your property.
Confirm that when you lower the ball cock in the cold water tank that no water FLOWS.
open all the cold water taps in the house to drain water out from the highest point (wait till water stops).
with rags ready just in case
1) Now saw through the pipe below the stop tap replace with a new stop tap and replace/remake the pipe up to the tank
2nd version.
just place a new stop tap in the pipe above the old one remake pipe to tank.
open the new stop cock all the way OPEN then CLOSE it about 3/4 of a turn this allows a bit of lea way that if it don't turn the one way you can always try the other way.
suggest you turn tap fully closed then open again to the above settings every couple of months to prevent SEIZING UP OCCURRING again.

my one finger is sore doing this much typing.

wirecutterman
January 2010