I have fitted a new thermocouple and the pilot light stays on - BUT - when the main burner come on it closes down after ten seconds or so.
I suspect a blocked pilot light jet but this is a new one on me. n.b. I've been running the boiler since 1982. It was probably fitted about 1971??
p.s. is there drawing of the metal flame divider which gives three flames - one for each burner and one for the thermocouple. I think the third one is not a big enough flame.

Alex, June 2010