its very unlikely there isnt anything to turn it off.
Its normally a gate valve (big red tap) in the airing cupboard or an isolation valve in the pipework leading down from the loft into the bottom of the cylinder.
Even if there isnt, you dont have to 'drain' the cylinder - just turn off mains to the CWST in the loft via stop cock under sink, make sure it has worked and the storage tank isnt filling, and turn on the cold and hot taps to all sinks until the water stops....
There will still be water in the cylinder, but no head from the loft to push it out.
Even easier there may be a ballofix type valve under you sink on the pipe run up to the mixer. If there is all you need to do is close this for the hot.
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