criminal conviction

Please help...
I am qualified under the HSMP, except I was convicted of a misdemeanor. The conviction was expunged and dismissed after I completed probation. Most records won't show that I was convicted, and I am not legally required to disclose the conviction on any job application that asks if I have ever been convicted of a crime. However, it is my understanding that some databases may still show the conviction. Does anyone know if a dismissed conviction will disqualify me, and how extensively the UK searches the criminal history of HSMP applicants?

Thanks!

Honesty is always the best policy. Explain exactly what happened and that it was cleared. Not disclosing it is not really an option, since if they discover it, say, when you come through immigration on arrival, you could be denied entry despite having entry clearance. It's just not worth it.