By turning off the cold supply and running the hot tap should empty most of the cold water tank in the loft, any water left in can be siphoned off using a hose.
To drain the cylinder, there should be a drain off at the bottom of the cylinder if not there should be one on the cold feed just before it enters the cylinder. Attach a hose and drain.
If there's no drain off, open the hot tap until the water stops flowing undo the hot water draw off and place a hose to the bottom of the cylinder then it can be siphoned, sucking the other end of the hose will get the water flowing.
Draining the heating system will empty the heating coil, the water in the coil may (probably) be black so blanking or bunging will help protect the carpet or flooring as it is a nightmare to clean up.
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