At this time in the UK there is no mandatory period that operators have to undergo refresher training, though Instructors have to undergo re-registration every 5 years.

Individual companies can have a refresher training period as company policy or if their insurance company insists on this.

The HSE publication (HSG 136)(workplace transport) states that the Health and Safety Executive strongly recommend that operators undergo a period of refresher training every 3 or 5 years. Technically if an operator fails a test after refresher training no-one has the authority to revoke his certificate, But the employer has the option of withdrawing the operators authority to operate their machines.

You have to ask yourself if your refresher training is aimed at the operator passing the test again OR does he have the skills and attitude towards safety to operate in the workplace.