This Mail Online report highlights the problems with not testing. More is to be found here. It would seem there is a list of failings.
1) Plasterer damaged cable.
2) Some one turned on supply without first testing.
3) Testing was carried out by more than one man so no one realised bits had been missed.
4) The boiler did not have a safety cut out if the thermostat failed.
5) The plumber did not glue a pipe.
6) In spite of noting electrics sparking did not turn off power first.
The supervisor who had semi-skilled men do some of the testing and entered his name as doing all the testing seems to have born the major part of blame. Also today either Ali-tube cables or RCD protection would prevent it happening but the fault could go for 10 years until first EICR before it was found even if found then had the boiler not leaked.
So DIY people who do not test could easy find them selves in court 10 years after the DIY job. The fact that the semi-skilled guy should not have been asked to do the work did not mean he was not taken to court. OK found not guilty but it must have been 7.5 years of hell for him not knowing what the court would say. Had he been skilled I am sure he would not have got away so lightly.
I am sure at some time we have all cut corners here is a stark reminder of what happens when it all goes wrong.