How do I get the tap cover off?

I have a shower tap which is a bar with Hot & Cold water connections. On the right hand side is the temperature control and the Left hand side has the shower control for the fixed shower head or the flexible hose.
The problem is the tap on the left is seizing up and not turning. Probabaly limescale [even though we have a water softener] How do I get the tap off. There seems to be a centre cap which may come off but I am not sure.

Alan Bagley
September 2011