I can't speak for this particular employer, but broadly -
Employers can't just decide to ask all applicants for detail of spent convictions and / or do an enhanced DBS check - the role has to meet certain criteria.
It's usually for jobs involving being directly responsible for children / vulnerable people and so on, as in working in schools. Not (for example) shop staff who will occasionally have children as customers. I'm not sure whether train conductors are included.
Does the application form (or web page) specifically say the job is exempt from the rehabilitation of offenders act and ask you to include spent convictions? If it doesn't, then you are not obliged to disclose spent offences.
If the job is exempt, then in general, failing to declare something that they find out about later is more likely to count against you than disclosing it now.
Unless you're on the 'barred' bit of DBS, i.e. have been barred from working with children, then it's not DBS's role to say to employer 'you may not employ this person because they have a record', it's their role to provide info to potential employer to enable them to make a decision.
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