Under the Rehabilitation of Offenders Act, nearly all criminal offences become 'spent' after a certain period of time (which is determined by the sentence imposed by the court). There are only three exceptions:
(i) convictions resulting in a sentence of over 2.5 years imprisonment are never spent;
(ii) no convictions are ever spent when the type of employment is exempted from the provisions of the Act. (That basically means jobs working with children, vulnerable adults or in the 'administration of justice); and
(iii) situations covered by the laws of other countries (e.g. applying for a visa to visit the USA, which does not recognize the Rehabilitation of Offenders Act).
(i) & (iii) clearly don't apply in your case. The maximum length of time for any conviction to become spent is 10 years so,
unless you're applying for a job which is exempted from the provisions of the Rehabilitation of Offenders Act (which must be clearly stated on the application form), you can legally answer 'No' to any question about convictions.
As Norman states, a CRB check will always show up all convictions (whether spent or not) but employers are only allowed to obtain CRB checks if the employment would bring the employee into contact with children or vulnerable adults. Most other employers are not permitted to carry out CRB checks.
http://www.lawontheweb.co.uk/rehabact.htm
Chris