Leaking single lever mono block mixer tap?

We have a tap similar to this link

http://www.tapstore.com/acatalog/Storm_mixer_inc_pop_up_waste.html

Water is leaking, not from the spout but from between the lever and the tap body. The tap is in an outside loo and started leaking in the winter. We cannot see where to gain access to the inside works of the tap.

There are no visible screws or pop off discs.