Great advice, thanks. The water's going in gently to try and minimise as much stress as possible for my fish.
Should I leave the tank when full for 24 hours and retest?
If you've got the time, retest after about 15 mins (to give the new water a chance to mix with the old).
Yes I have but looks like I'll have to go out and buy some more Aquasafe at some point.
A lot of people recommend Seachem Prime, becuase it's very concentrated, and therefore lasts longer.