Unless it resulted in a prison sentence of more than 2� years, any conviction from 23 years ago will be 'spent' when applying for most types of employment. You can lawfully answer 'No' to any questions about whether you've ever been convicted of an offence. However, if a CRB check is to be carried out, that might be unwise since an employer can lawfully sack any employee (without the need for any reason, and with no right to claim for 'unfair dismissal')) within the first 12 months of employment, so it would be best to be honest from the start.
When applying for those types of employment which are exempted from the provisions of the Rehabilitation of Offenders Act, convictions are never regarded as 'spent' and must always be declared (even if not apparently relevant to the employment). If a subsequent CRB check discovered an undeclared conviction the employee could face both dismissal and criminal prosecution for 'fraud by false representation'.
Chris