The whole point about a conviction (or caution) becoming spent is that you're allowed to lie if you're asked about whether you've got any convictions (or cautions). So you're never required to declare anything which is spent (other than for applications for certain types of employment where the provisions of the Rehabilitation of Offenders Act don't apply).
The rules about convictions/cautions becoming spent also apply to 'ancillary matters'. For example, someone with an unspent conviction might be declined insurance. Later, when the conviction has become spent, he applies again and he's asked "Have you ever been convicted of a criminal offence?" AND "Have you ever been refused insurance?". He can lawfully answer 'No' to the first question because the conviction is spent and he can also lawfully answer 'No' to the second question (because it relates to an 'ancillary matter').
Chris