to power flush or not to power flush

Hi i have a glow-worm compact 24 combi the dh went in Febuary so called a guy in who fited new d heat exchanger its now gone again, had BG round and they wont touch it (cant get parts) BG say we need another heat exchanger also to power flush the system. Ive bought new heat exchanger and i feel i need to power flush the system but not sure how to, do i only do the rads & pipes or do i do the combi boiler before i fitt the new d plate also do i need to have the system running hot while i do it could some one explain in laymen terms how to do it also will this stop the heat exchanger going again or do i need to do somthing else aswell
Andy

Personally I would only power flush the Heating cirucit and Rads i.e. with boiler disconnected

Why would the dh heat exchanger have gone in only 5 months ?

What were the symptoms that led to the replacement of the DHW heat exch? it may have been sludge, but that would also impact on other comonents such as diverter valve, it may have scaled up if you are in a hard water area and you have no softener or scale reducer but 5 months for a PHE is a very short life