Don't know where you are in the UK, but these plants are extremely hardy and shouldn't have been particularly affected by the winter. If you're anywhere in the south, it could be drought. You also don't say whether the plant is in the ground or a pot - water if dry, repot if necessary, feed with something like Miracle Gro general purpose, or just Growmore raked around the base. This assumes you've inspected the plant stems and leaves thoroughly for signs of infestation of any kind.