Almcd007's answer isn't entirely accurate. (In fact, it seems to be back to front!):
Courts are not obliged to disregard an offence which happened many years ago when considering sentence. I.e. as far as the courts are concerned, a conviction is never 'spent'.
However, once a conviction is 'spent', you do not have to declare it on any employment application (except where the type of employment is exempted from the provisions of the Rehabilitation of Offenders Act). You can lawfully write 'No' in answer to the question 'Have you ever been convicted of a criminal offence' without fear of prosecution for 'seeking to gain a pecuniary advantage by deception'.
Paul's link provides the information you require about the Rehabilitation of Offenders Act. However, Almcd007 is correct in stating that a criminal record is for life. The Rehabilitation of Offenders Act simply means that, for most purposes, you're not obliged to declare your convictions after a set period of time.
Chris