Variable hot water - Vokera Excell 80SP?

I have an issue whereby when in the shower, the hot water varies in temperature. Every few minutes the heat drops for a minute or so and then comes back.

Just had the boiler serviced and the engineer said that the issue was sludge in the heating system. This was causing a blocking in the heat exchanger (the central heating one?) and because the water wasn't getting through the whole system (presumably the bit in the boiler before it is routed to water or heating?) temperature was building and the burner was cutting out.

He suggested a power flush and replacement of the heat exchanger. I wanted to know if it is the central heating exchanger that needs replacing? Presumably, as that one is the one blocked - although it is the hot water that is being affected.
Any advice gratefully received.