faulty fire help please

[gazwhyte1]

I have been out to look at a jetmaster, chatsworth mk6 fire today that is faulty just after a bit of guidance am used to the installs side but branching out into the faults,
anyways was all working up until today.
the pilot lights but does not open the main burner, the pilot does however stay on in the high flame setting?
I was thinking the thermocouple not opening the main burner if anyone could help with this would much appreciate it, is there a procedure for fault finding on the themocouple and gas valve
many thanks
gaz

 

[dcawkwell]

Replace thermocouple they are cheap. If that doesn't fix it.
Gas valve. But most likely will be thermocouple.