The pilot light on the above boiler would not stay lit and I have replaced the thermocouple with a HSS-001 Universal Thermocouple length 900mm. The existing one was 600mm. The pilot light still won't stay lit. I followed the instructions for re-lighting the boiler, i.e. everything switched OFF, electric supply, thermostat controls & timers, but still the pilot light still won't stay lit. Am I missing something? I believe the thermocouple is the usual source of the problem when the pilot light won't stay lit. Any suggestions?
Flipflop, As you state, the thermocouple is the usual source
for this problem, but alas, it is not always the case.
It is possible that the thermo-electric valve ( which is incorporated in the actual gas valve ) may have failed.
Suggest you acquire the services of a corgi registered engineer.