at the top where the water comes in at the level of the ball device. a fine spray of water was coming out.
draining down and gradually turning back on seems easy enough but will it definitely work?
I know hindsight is a fine thing, but the entire tank didn't need draining down to repair a leak at the top of the tank.
Getting rid of an airlock in hot and cold pipes from a tank can be a tricky job, it's not easy for me to explain on here without seeing the tank, pipes and what type of taps you have. And I have to go to work now, sorry.
I would bite the bullet and ask your plumber for advice, not sure how well you know him.