Dear (recipient's name),
I am writing with reference to your complaint on (date). I apologize for the inconvenience created by our error. We take great care to ensure that important matters such as this are properly managed. Due to construction, occasionally our high standard is not followed, and clearly this is the case with your situation.
In light of this, we have decided to (solution), which we hope will be acceptable to you, and hope also that this will provide a basis for continuing our relationship. I will call you soon to check that this meets with your approval. Please contact me should you have any further cause for concern.
Sincerely,
(your name)