Do the 2 heat exchangers in my combi boiler affect each

Sorry for the later reply - I was in all morning but out all afternoon.  I will read your posts and then reply.

Firstly, the heat exchanger cleaning that's referred to in the service instructions has nothing to do with the water pathways through either of the heat exchangers.  It's all about cleaning the combustion chamber and the surface of the main heat exchanger where the heat transfer starts.

To answer your other questions:

1 . What happens to the boiler if the installer fiited the magnetic filter to the feed pipework saying it didn't matter feed or return?
That depends on whether or not they followed the installation instructions for the magnetic filter.  I would expect those instructions to recommend fitting the filter on the return, but I don't know that for a fact.

2. Another engineer changed pipework to the return? Can you explain what difference it makes please?
The particles that the filter is designed to trap comes from the radiators, so the idea is stop those particles from reaching the boiler and baking onto the surface inside the heat exchanger.

3. Hard water area : Should he have fitted a separate limescale filter too?
If the boiler manufacturer recommends it for a hard water area, then yes.  Or, if in the opinion of the installer it is warranted, then yes.

4. Would it make a difference?
It depends on the product fitted, and the actual hardness rating of the water.

The usual method of replacing an expansion vessel inside the boiler casing is to isolate and drain the boiler.  Hardly any primary water is lost.

It feels to me that you don't really trust the person who you're paying to maintain your equipment.  If that's the case then you should not use them, but get someone you do trust.

It seems a bit cheeky to fail to maintain the system, but then recommend a costly power flush to remedy the result of their lack of care.  You need to find a heating engineer who will look after your best interests.

"Why would it" is a question for whoever claims it's blocked.

5. "They want me to have the system power flushed which costs £400/500"
Flushing the system includes the radiators, so they are saying that the system is not fine.  The system includes the radiators.

6. "...a plate heat exchanger is a £50 part easily changed?"
The plate heat exchanger is for the hot water, and does not serve the radiators.  

7. "Why only the boiler gets the problems of blocked plate heat exchanger?"
If you're talking about the plate heat exchanger, it can scale up because it has fresh water passing through it, which has nothing to do with corrosion, or with the radiators, and cannot be removed by power flushing. A new one costs around £90, not £50. In many boilers it's not a lot more.
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Again, all of your questions indicate that you don't believe the information being given to you by the engineer you're paying to maintain the system, who should be aiming to reduce your total cost of ownership.  If you don't trust them, then ignore what they're telling you and get someone better.

You're welcome. I'm pleased to hear you are another Happy Customer. If you would like to ask me another question in the future, you can add me as a favourite Expert.

Have a great weekend :-)