The judge will decide whether or not the old conviction should be disclosed in open court. You should ask your barrister about this.
In terms of sentencing, the judge ought to sentence you as if he were doing so at the time of the offences, i.e. with you having no previous convictions, and with the sentences applicable under the laws at the time, not under current laws (if they have changed in relation to your offence), but it's hard to know. Again, this is for your barrister to advise.
Sadly the UK is a global outlier in terms of not having a statute of limitations for criminal offences at Crown Court level.