Harry Bloomfield wrote:

None presented the following explanation :

how would I drain down HW system btw?

Turn the water off, where it goes into the header tank or tie the float
up - then turn all the taps until the all go dry. But make sure there is
no source of heat into the cylinder first.

That'll leave the hot cylinder full, look for a drain point at the base
of the cylinder where the cold enters, or failing that you may need to
undo the threaded connection at the top where the hot exits, and syphon
it with a hose.