Tosh, your red book is only as good as the day it was issued...it details your army career to the point of discharge. I left December, 1988 and could have committed a string of offences between then and now with there being no reference to it on my 17 year old discharge book.
The ONLY proof of a persons criminal past (or lack of) is to apply for and pay for a CRB certificate.
If you were working inside an establishment where you had unsupervised access/contact with children or vulnerable people/ cash/valuables or drugs you would have to provide proof of an enhanced CRB and declare ALL convictions whether spent or not.
If the offences are deemed to be of such an age or of no relevance to the risk they will not appear on the certificate. Any offences against the person or theft will always appear. Once you receive your certificate, you can then decide, based upon it`s contents, whether you can then use it when approaching potential clients for work.
If the client has requested you provide a certificate, they will (or should) be a registered body with the CRB and have authorisation to carry out such checks and check all relevant ID documentation you will need to supply to verify your identity and address history from the past 5 years.
They will complete the paperwork and are entitled to ask you to pay the fee. A certificate will be sent to both them and yourself.
The decision to employ or not will then be made.