You have two sepearte issues, the pressure rising in the CH system is due to the expansion vessel needing recharging/replacing.
The HW issue is something you must have had 3-4 years ago to have it replaced? Debris from the CH system is clogging up the DHE causing your boiler to keep going out (the heat being produced can't get cleanly through the DHE), you can fit a new DHE again, but without cleaning the system, the problem will re-occur again some time down the line.
Carefree
February 2011