I did Google prior to this post but was a bit confused by a variety of
answers.

I have a gravity fed CH system with standard boiler with a programmable
timer. Temperature for heating is set by those commonly found Honeywell
mechanical large-dial room thermostats. I work it so the CH comes on and
off when I want and when it is on, the room thermostat controls the
temperature. i.e. boiler runs until room temperature matches set
temperature on thermostat. If CH is not in an 'on' program, room thermostat
does nothing.

It all works fine except it would be great to have more temperature control
so the heating could be on low overnight or when the house was unoccupied
and rise just before waking up or coming home. I am hoping that replacement
of the mechanical thermostst with something like the electronically
programmable Drayton Digistat 2 or 3 is the answer?

Is this just a simple swop over for a novice? I am seeing references to 2
or 3 wire systems and wires having various controls of the boiler. Surely
the mechanical system I have now simply sends to the boiler 'below
temperature = on (makes circuit)', 'at or above temperature=off (breaks
circuit)'? Doesn't the end result of the pragrammable version just do the
same?

I was thinking it would be simply an easy case of swopping over the boxes?