how can i find out if my potterton ep2001 programmer is u/s?

I belive my potterton ep2001 programmer is nackered as it will only put the central heating on twice a day no matter what selection i have i.e constant. I do have a thermostat switch which in the past has allowed me to have the programer on constant and i have been able to control the heating from that However it now has no effect and the potterton seems to be stuck on a preset auto setting. Is there anyway i can test the programer to see if it is that or my thermostat switch(british gas PT5) which is at fault