On 07/10/2020 16:52, Jimk wrote:
Hmm to clarify, all of those valves are somewhat crusty &
unwilling to operate properly & I fear things could go t1ts up in
a trice....

Lie a length of 2x1 across the top of the header tank and tie the float
valve up to it to stop the tank from refilling. Then turn the tap on and
the flow will stop when the tank runs dry. You can reduce wastage of hot
water by baling out as much as you can from the header tank into a bucket.