This came as something of a shock, to both the customer and myself. The heat exchanger (the finned chap at the top) had obviously been leaking mildly for some time and rusted out the casing. New heat exchangers are available, however in this case because the casing and burners had been damaged the only solution was to condemn the unit. Since regulations now forbid the installation of these relatively inefficient devices, we had to rework my schedule in order to install a new combi boiler for the customer as soon as possible.
As an aside, the only time apart from a service that you might get an inkling that there's something wrong is a pool of water appearing on the hearth in front of the fire. We were lucky it didn't get that far!