Thank you for the replies. Anne, I may do that if I am sure that all of the damage is caused by the fertiliser burn.
Baja, there are holes at the bottom of the container but they seem to be ineffective because water does not drain. I have made sure the soil is not tightly packed around the holes but water still does not drain.
The fertiliser burn was caused when I poured some almost undiluted Powerfeed into the pot. The leaves that had drops of fertiliser on it died. A few days later, it was watered with dilute Powerfeed, I don't know if this caused any more damage.
It has not been fertilised at all since this happened about 3-4 weeks ago.
The other leaves started to die about 2 weeks ago.
Ingredients of Powerfeed can be seen below: