faulty timer on j15-22 Johnson & Starley??? (timer CL25 24hr)?

hi,

my j15-22 Johnson & Starley heting system does not come on when using the timer.
When "on all time" is selected the heating system runs fine.

When the timer is selected, there is a click sound when it changes from a preset off period to the on period. However the system does not 'fire up' and heat the house.

Anybody know the cause?
is it likely to be the timer itself or something more costly?
Also if it is the timer, i could not see clearly how i change this.

any advice appreciated.

andy