As you state, a caution is not a conviction. There are currently effectively two types of caution, a conditional caution and a 'simple' caution.
A conditional caution (providing the conditions are complied with and there is no subsequent prosecution for the offence) becomes spent three months after it is given and all other cautions become spent when they are given.
As conditional cautions are a relatively recent invention I suspect that you received a simple caution and this became spent the moment you received it. The caution would remain on your record and had you committed a further offence within a period of two years the fact of the caution could have been used in deciding whether or not to prosecute.
If the form indicates that you do not need to disclose spent convictions you will not need to disclose a spent caution.