There is no such thing as a 'icence' to drive a fork lift truck. There is simply an obligation upon employers to ensure that employees operating lift trucks have received suitable training. (Any document received, as a result of such training, is simply a certificate of competence, not a licence per se).
The Health & Safety Executive states:
"There is no specific requirement to provide refresher training after set intervals, but even trained and experienced lift-truck operators need to be re-assessed from time to time to ensure they continue to operate lift trucks safely. In addition to routine safety monitoring, re-assessment might be appropriate where operators have not used trucks for some time, are occasional users, appear to have developed unsafe working practices, have had an accident or near miss, or there is a change in their working practices or environment."
http://www.hse.gov.uk/workplacetransport/lift-trucks/faqs.htm