plate heat exchanger?

for the second time in 2 years i think my plate heat ex in my vaillant combi is clogged up. the Ch seems to work ok but i cant get the DHW to to get hot. when i run the hot taps the water temp fluctuates between tepid and cold. i have put some sludge remover in the Ch system via a rad bleed valve. When i drain and flush it out will it improve my plate heat ex performance ?or do i have to look elswhere for the problem

ian H, bingley
January 2010