Refill and bang in some inhibitor

IANAP (I am not a plumber!), this is just how I would personally approach
it!

Toby...

Just to add to this, I would also refill, drain and refill again.

Check for leaks, then when you are 100% sure all is OK, add the inhibitor.

If you have any motorised valves, then say so before starting!

Toby...