Write a letter complaining about improper work.

Dear Sir/Madam,

This is to inform you that on (date) I contracted with (trader) for (nature of work). This work was financed by a credit agreement with your company (agreement number).

However, it is with regret that I have to inform you that I have discovered a problem with this work, namely (list problem). Therefore, I have requested the trader to inspect the work and make the necessary arrangements to carry out remedial measures. I would appreciate if you associate with the trader (name) and ensure that the work is carried out at the earliest. 

I shall look forward to hearing from you within a week's time to confirm receipt of my complaint.

Yours faithfully,