Only notice this problem thanks to obdeleven scan, I had a look at the cap but there is not a screw to undo to remove the cover to get to the actuator, how do I remove this ? 

Hi, there are four slots, two top & two bottom which you need to pierce and put in a head unit removal tool/ flat screwdriver to lever and pull the whole thing out. 
I had mine Jammed closed 🤬

OK thanks for that, I will try it over the weekend if/when I get free time.