Hi, I am quite suprised that the cylinder was not upgraded to comply with the energy efficiency regulations when they installed your boiler because this should have done. Any how it obviously has not been done why i cant say !!! but i do not think scale is the problem considering all was well before the new boiler was installed. I would think that it is either bad plumbing techniques and air is accumilating in the feed pipes causing a slow delivery and will need to be bled out or blasted through, or a pump failure or the by-pass has not been set correctly and my need adjusting accordingly. Also the boiler thermostat may be set too low or the cylinder stat set too low both should be set to 60*C.