If you read the UKBA guidance carefully, there is a requirement to mention arrests as well if you have not been told by police that you wont be charged. In your specific case, if you were issued a simple caution, then it is regarded as spent immediately. If it was a conditional caution, then it will take 3 months to spend from the date conditional caution was issued. So please check the paperwork you received from police.
You actually do not need to disclose a caution once it is spent, but in your case, since it is in the knowledge of UKBA, it would be a good idea to explain everything in a covering letter and also include any police paperwork.