Write a letter complaining and about faulty item and requesting a replacement.

Dear Sir/Madam,

This is to inform you that on (date) I bought (item) from (retailer). This purchase was financed by your company on agreement number. Unfortunately, I have discovered that this (item) is faulty and not upto standard. In fact, I have written to the retailer to request the necessary repairs. I would like to inform you that under the Consumer Credit Act you are equally liable with the trader for any such unsatisfactory goods and I, therefore advice to liaise with the (retailer) so that the matter can be resolved at the earliest possible. 

I shall look forward to hearing from you within the next few days. 

Yours faithfully,