Actually; it's very simple: call the regional commercial manager [the number is on your bill]. Ask to speak to the manager or his/her assistant. Then follow-up with a letter delivered to their office. You will see things moving.
If that doesn't work; call the HQ and ask to speak to Eng. Rosemary K Gitonga, Chief Manager, Commercial Services....or her assistant... now that will get things moving very quickly!
NB: make sure you have details at hand - acc no., dates paid, amounts paid etc
Nothing is real unless it can be named; nothing has value unless it can be sold; money is worthless unless you spend it.