Thermocouple failure is very common Alan, as you've obviously found out. I guess it's still possible that the new one has failed.
The trouble is, there are so many little sensors,thermistors etc that can stop a boiler from working.
I guess your's is the type where the t/couple keeps the solenoid open to allow gas in.
I'm no boilerman I'm afraid. Silly question, but is the t/couple in the pilot flame properly?
I remember coming across one where it was out of place, thus never quite warming up.
Otherwise.......... gas valve... even a circuitboard fault.
Sorry that's all I can suggest Alan. Maybe one of Answerbank's proper boiler guys may see this. If not, probably call-out time.