The EAT has ruled that:

• A worker must be able to take annual leave within a period of 18 months of the end of the leave year in respect of which the annual leave arose.
• A worker does not have to show he/she was unable to take the leave in any year - he/she is not required to take annual leave but may choose to do so or not to do so.
• A worker who does not wish to take annual leave during periods of sick leave is entitled to take the annual leave at a later date - within 18 months of the end of the leave year in which the leave accrued.

The claimant in this case was a plumber who was off sick for almost four years, from April 2010 - Feb 2014. When he asked for annual leave in his final leave year of 2013, the employer agreed but refused to pay for previous years. The EAT ordered that the claimant was entitled to payment in lieu of holidays for 2012/13 but not before. http://www.bailii.org/uk/cases/UKEAT/2015/0071_15_0807.html