Worcester 350 Combi Plate heat Exchanger - Is engineers diagnosis correct ?

We have a Worcester 350 Combi and recently when we have been running the hot water the system has been overheating and has shut down. - The system is fine when the central heating is working.

We have had to reset the system by the usual method of presssing the overheat thermostat reset button on top of the boiler.

We have had 4 engineers come to the house this week to look at the problem. The first two could not replicate the problem so did not do anything.

Then we were able to identify that the system predominently shuts down when we were either running the bath on the first floor or having a shower in our ensuite on the 2nd floor (we live in a three storey town house). We called again and told the third engineer about this and he was at the house about three hours and could not put his finger on the problem. The system would not shut down when running the water on the ground floor in the kitchen where the boiler is located, but is cutting out on the upper floors. He tried to suggest that the pipes were possibly blocked thereby causing the heat exchanger to overheat. I explained that the bathroom upstairs is less than three years old and has all new pipes. So he left without being able to identify or resolve the problem.

For the last two nights we have tried to run the bath and the shower with the taps only turned on about half what they normally would be and the system has been working fine - no shutting down at all.

This morning another engineer has been (now the 4th) and I showed him our findings to prove that the system is ok at half tap turn, however, he has concluded that the plate heat exchanger is overheating and the likely reason is that it is probably blocked with magnatite - magnetic particles from the system that are sticking to the plates and thereby causing the plates to become blocked and as a result the heat exchanger is not working properly and overheating and not dissipating the heat correctly.

The problem is that our insurance does not cover the cost of replacing this part - the clause in the insurance policy states - "descaling and any work arising from hard water deposits or from damage caused by agressive water or sludge reulting from corrosion. Signs that work is needed may include a noisy boiler sludged up pipes or poor circulation".

What I would like to know is does this sound like the problem has been diagnosed correctly as it has taken 4 engineers to arrive at this point. We are unhappy that it has taken over a week to get to this point but as we are not experts it looks like it is now going to cost us a lot of money to get this fixed.

The engineer is also suggesting that we have somehting called a magna clean fitted to the system next to the boiler to take away any magnetic particles as if this is the problem then he is saying that the problem could just reappear.

Any help would be greatly appreciated.