@chenks: sorry, but that's rather patronising. Of course I don't confuse flow temperature with room temperature. But a lower flow temperature means a lower radiator temperature, which means less heat output. The system will take substantially longer to heat up after a cold night, just when I need it the most.
not substantially at all if it's set up correctly.
the room will take slightly longer to get to desired temp, but not substantially longer. if it's taking substantially longer then you either haven't got the boiler and rads configured and balanced, or you have a heat loss issue in the room(s).
for comparison, i have recently adjusted the flow temp on my boiler, not for any cost saving reasons, but i've moved house and took the opporutinity to replace some rads that were too small and change the boiler from relay mode to opentherm (modulation) mode.
change the flow temp from the max 80 degrees down to 65 degrees (which should be in the range for condensing to start kicking in - anywhere between 60 and 70 should do it, even though some suggest 50 which is usually bad info). i've noticed no perceivable difference in the time it takes to heat up the main room to 21 degrees in the morning after the heating has been completely off overnight (ambient temp seems to be around 60 degrees). the stat then is set to keep the room at a steady 21 thru the whole day, and the boiler kicks in every now and then, once up to temp, to keep the temps at that.
with relay mode the boiler would kick in at full power any time the stat requested heat from the boiler, in modulation mode the boiler only powers up to what it needs to top up the heating, so not the full 28Kw, but maybe 10Kw or whatever it needs at the time.
not all boilers support modulation though, so depends what you have.
mine is an Alpha Etec 28.
working for a gas central heating installation and maintenance company helps though (although i'm not in the gas dept), so i have access to people who tend to know how to set up things up.