Condenser Boiler Hot Water Problem

Hi, I wonder if anyone has any ideas, advice or tips.
I recently had a new Bosch Condenser Gas Boiler fitted with all the controls which qualified for the SEAI Grant.
Central Heating and Hot Water work independently of each other.
All is working well with the Central Heating.
However, my problem is with the Hot Water.
I had the fitter out who checked the system and said all was in order.
Whether it is running simultaneously or independently of the Central Heating the water will take at least an hour to heat and the temperature won't be hot enough for a shower.
I have a lagged medium sized copper cylinder with a thermostat sited and set correctly. If I use the electric immersion it will take 15 mins to heat a sink capacity to piping hot.
Would anyone know
a) if any hot water should access the cylinder when the Central Heating Only is on?
b) if it is more cost effective to run the Hot Water at the same time as the Central Heating as the boiler is already on and water heating anyway?
I understand the benefit of a dual control system as presumably the energy consumption would be higher with all systems on at once and the fact that to have hot water you wouldn't have to turn off the rads when they're not needed.
But presumably it makes more sense to have hot water when the system is running for central heating anyway.
Thanks.

What is the output of the boiler?
How many radiators do you have? (Doubles & singles)
How size is your hw cylinder?
Does the hw heat to your satisfaction when the boiler is heating hw only?
What temperature is the cylinder thermostat set to? (It could be a faulty thermostat or incorrectly wired).

To answer your specific questions:
a) No. When central heating only is on, then this will only heat your radiators and no heat whatsoever will heat your hw.
b) If the output of the boiler can handle both on the same time, (and it should be able to) then it is more cost effective to heat both at the same time, but only heat hw for a shorter time. The cylinder is thermostatically controlled anyhow and this will turn off the motorized valve & the boiler when the required cylinder temperature has been reached.