Hot water problem in my combi boiler?

My combi boiler works fine in central heating mode.
When in hot water mode, however, it fires intermitently so the water gets hot,cold, hot, cold and so on.
BG insisted that I need power flushing because the secondary heat exchanger is half hot, half cold, therefore it is choked by sludge.
They also said they can replace it for a cost of around £250.
I noticed that it can be easily pulled out by unscrewing two screws.
Can I clean myself?
Thanks in advance for replying.