I will try to help not confuse.
Gate valves are carp IMO when they get old they are even worse.
I am beginning to see this I have heard similar from the plumber who came over yessdi and it is all over online how carp they are.
Older systems were laid out so as to have mains pressure to the Kitchen tap/utility room (maybe). The rest of the cold taps were fed from a tank. This tank would also feed the hot water tank, it would enter at the bottom of the hot water tank.
Yes this is the case.
After you drained your cold water was the tank empty?
No. It just drained the kitchen tap, the outside tap and the utility tap. The other taps still ran, so I know they are fed from the header tank. It just took an amazingly long time. I guess it drained all the pipes that feed the tank in the roof, and maybe the one with a ball valve to the little header tank for the CH.
If not then your taps you drained were all main supplied taps. If it took ages to drain then I would expect them to be fed from the top tank, if this is the case it should have been empty when you looked.
If it was still full then you could have just run a hot tap to feed the hot tank and empty the top tank, as soon as the top tank is empty then no more water can come out of the hot tank as its fed from the bottom and tap supply is from the top. This then would mean you only have to turn off the mains and run a hot tap, no need to mess any more valves.
If I can't get the correct gate valve to work then this is what I'll do, and for good measure I'll tie up the ballcock so that it cannot feed the header tank.
But yes central heating is more urgent need right now good luck
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Thanks mate!