Well, I am sure that they would be delighted to receive a letter from you - customer's are so quick to criticise, but so slow to complement when required.
Was there a specific person whom gave you excellent service? If so, you can mention them by name and find out their line manager's details to write the letter to.
Something along the lines of:
"...I have had numerous problems with my account in the past, and X could not do enough to rectify the situation at hand. They were polite, helpful and ensured that all my needs were catered for in a professional manner...."
I hope that this helps and that you do find the time to compose the letter.