Sorry for the delay in responding. The question of night setback does come up. The good news is that you can use night setback thermostats with mod con boilers and you will see some savings. Here is the impact of using night setback.
1) It will take slightly longer to bring the house back up to temp when using outdoor reset as compared to traditional cast iron boilers. This is obviously due to the lower supply temperature that the mod con will usually have. This will especially be true when using radiant flooring.
2) You will lose a small amount of efficiency while bringing the temp back up as the boiler will be running at a higher firing rate. This should be rather small but it will be there in theory.
My advice is that I would use the night setback feature of the thermostat, but recognize that you may need to set the recovery time a little earlier that you may have thought. Anything that can improve fuel efficiency should be tried. Each home is different so we can’t give specific savings amounts, but I would experiment to find the optimum settings for your house.