Dear Sir/Madam,
This is to remind you that I wrote to you earlier on (date). I am also enclosing a copy of that letter. However, I have not yet received a response.
I have given you reasonable opportunity to resolve my complaint but you have failed to do so. I would like to assert that unless I hear from you within 5 days I will have to resort to court action. The costs of this action will be added to my claim.
Yours faithfully,