clock problem on vaillant combi boiler?

I have a Vaillant Turbo Max VUW BB242-5
The problem is the the central heating doesn't come on or turn off, like it should using the clock/timer.

I guess this is a fault with the Clock? Would someone be kind enough to confirm this and let me now where I could buy a replacement if this is the case. I can remove/replace the clock from the front of the facia no problems.

It is about 7 years old.

Thanks in advance

Andy Doherty