My Worcseter Bosch 42 CDi Combi Boiler?

Just got this fantastic boiler installed. However had to change from conventiional system to combi system and instead of cold water from overhead tank now using the mains water for boiler input. Also , had to redo entire piping - replaced steel tubes with copper. Now the DHW pressure is not as high as the water main when a tap is open. Can it be solder flux in the boiler piping ?? How can I match the HW outpir pressure to atleast or equal to Water Main inlet pressure ?? Any guidance , suggestion or help will be appreciated..