I was called out to replace this boiler today.  It is only SIX YEARS OLD!  As you can see from the photos, the flue is in a terrible state, completely corroded.  Unfortunately for the home owner, whoever installed this boiler didn't fit the flue correctly and over time, water has made its way along the flue and done all this damage.  Not only that, but there is a lot of damage inside the boiler too, especially the fan which is also badly corroded.

If this boiler had been serviced annually, the problem with the incorrectly fitted flue would have been picked up right from the start.  Also, the service engineer would have checked for any signs of water damage or corrosion inside the boiler.  Now the homeowner has had to bear the cost of replacing his boiler, when he should have been able to get many more years of use from it.  

So remember, it's always a good idea to get your boiler serviced!