Normally with these boilers you have a round mechanical timer. These have little tappits round the outside that you either push in or leave where they are.If i remember right in is for on and out for off. Then you have a little lever on it that you alter . it has 3 settings , the middle is normally off . If you put it to correspond with a little clock sign, thats on timed, then the other one thats left is continuous.
You can still get parts for these boilers, but a little tip , the hot water is normally very slow to come through, and there is a delay on the central heating when water has been used. Hope this helps