pilot light will not stay alight

[weargas]

as ollski said the black deposits are probably soot. If your heat exchanger is partially blocked by soot than the products of combustion could have done one of 2 things.
1. if they don't escape up and out of the flue then they can smother and suffocate the pilot light causing it to fail

2. if they don't escape they can cause a lot of heat to be retained within the casing causing damage to the overheat thermostat.

because you can't get the pilot to remain alight it's more likely to be the latter.

by all means change the the thermocouple because if it isn't that then at least you've got a spare but if it still doesn't stay alight then it could be the overheat thermostat or even the mag unit on the gas valve.

I would have a pro look at the combustion on the boiler though if its a damaged overheat stat.

 

[Brian4830]

the final result.
i got a new thermocouple(four pounds fourty four pence ) and the over heat sensor that is attached to it (thirteen pounds twenty pence)

having taken the old thermocouple out it could be seen that the end had been either burnt away (perhaps because the pilot Bray had been burning wrong due to its corrosion) or just its age
i decided to replace the sensor following advise in the forum .
all is now perfectly well and running ok .

i think i have learned that in future i will, at the end of winter , be turning off the gas and water to the heater , i base this on the fact that the above damage is probably due to the pilot burning constantly 24 hrs a day , even through summer, causing excessive wear and tear which is unnecessary as i only use the boiler about three months a year, thus i have cut the life of the parts by 75 per cent .
thanks for all the advise above , very much appreciated,
Brian