Write a letter complaining about work which had been contracted.

Dear Sir/Madam, 

This is to inform you that on (date) I contracted with you for the  (nature of job) and also paid you (amount) for this work. However, I am sorry to inform you that I have discovered a problem with this work, namely (list problem) and I am, therefore requesting that you make arrangements to come and inspect this work and make the necessary arrangements to have the necessary remedial work carried out as soon as possible. 

I shall look forward to hearing from you within a week's time. 

Yours faithfully,