Yes, you should disclose it. If you do not and they find out about it then they will view the answer as dishonest which could potentially effect the application more than the actual conviction. They are permitted to consider the conviction when making an assessment of whether you are of "good character" and you are going to have to take your chances with whether they do consider it to be influential.
For what it's worth the conviction would become spent after five years following conviction and if you have not committed any further offences then this will help. It will not be viewed in isolation and will be considered in the context of your entire application.
Please note that form SET(M) is for applications made whilst you are in the UK.
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