I’d say shut off the electric to the cylinder then shut the water supply to it off
Either by tying up the ball valve in the loft of it may have a shut off ‘tap’ where it enters the cylinder
Run the hot tap to empty the cylinder (no inlet so it won’t fill up)
Then I’d guess all your left with is any water in the pipes between the cylinder and the back boiler?
Edit
Is the back boiler feeding rads?
If so those 2 pipes coming from the back boiler I guess are are flow and return for the radiators