It happens that Jimk formulated :
Hmm to clarify, all of those valves are somewhat crusty &
unwilling to operate properly & I fear things could go t1ts up in
a trice....

As the cylinder is only on the floor above I was hoping for a
"limit the spillage" plan rather than a "potentially make tons
more work & inconvenience" plan.

If it is a cylinder fed from a higher tank, find something which is a
tight-ish fit in the pipe from the tank, to the HW cylinder. You can
actuall buy bungs for the job. No water going in the cylinder, from the
tank means no water coming out.