Why has Heat Exchanger on IDEAL ISAR HE24 gone?

So...

At the end of 2008 (early December, so about 4.5 years ago). We had central heating installed for the first time. An Ideal Isar HE24 Combi boiler. Not long after we had to replace the PCB (a well documented fault with the old black PCBs meant it burnt out. We now have an orange PCB).

About four weeks ago the DHW stopped working on our Ideal Isar HE24 Combi Boiler. CH was working fine, albeit a little erratically - starting up despite the connected remote thermostat being off (Siemens RDJ10RF).

We got a good Plumber recommendation, who came out to look at the system.

He diagnosed a knackered diverter due to the system not getting cleaned out since installation (Yes - I've learnt my lesson there).

He replaced the diverter and the system seemed to work fine...for a day!The DHW went again.

The plumber suggested it was the level of sludge in the system that was causing the diverter pin to stick, so he came and completed a full flush of the system: putting in cleanant, leaving it for a couple of days, returning and flushing the system til clean, and then adding some softener to stop the sluge build up.

However, six days later the DHW went another time. Water pressure had dropped below 1, and this time the PCB showed the error code L9, which from the installation/service manual and online forums is a possible block in the heat exchange (or a pump problem).

On the morning of 28 May (after advice from plumber and online forums) I simply turned the boiler off, topped up the water pressure, and reset the boiler. This worked and we got hot water once again.

But then overnight from Friday 31 May into Saturday 1 June, the boiler stopped working, again with an L9 code. When I tried to reset it nothing happened, apart from the L9 code changing to the three lines on top of one another (like a 3 without the side bar).

Later in the day we discovered a significant leak from the boiler, which yesterday the plumber identified as coming from the Heat Exchanger.

So...clearly we have a significant (and costly) problem. Here are my questions:
1. How much might we look to spend on a new Heat Exchanger?
2. How long does this job typically take?
3. Why might this have happened?
4. Might the other works have caused this problem?
5. Should this problem develop in less than 4 years on the boiler?