P38 heater blower box problem

P38 1998 Range rover 4.0SE auto

Problem - Will only blow air on to the front screen regardless of what button I press.

the heater control panel appears to be working ok, i.e. all buttons light when pressed, display shows temp etc

How hard is to gain access, i was thinking is there a solenoid in the blower box that could be sticking or could it be something simpler?

any suggestions?

regards

wayne