Back Boiler - Pilot goes out ...

Got an issue with my gas back boiler thingy. It's a Potterton Fireside "black Beauty" FWIW - in the house when I moved in, about 12-14 years old.
It had the same problem about 2.5 years ago - basically the pilot light goes out - re-lights easyily enough - runs for a while - hours to weeks, then goes out again. Eventually gets worse and worse. Got a man in and he changed the thermocouple, but even then it still goes out every now and then - until today when it seems back to hourly...
Any clues of what I can do myself before calling a man in? It looks relatively straightforward from what I can see - I can identify the pilot feed to the manifold, as well as the thermocouple - I'd not hesitate to replace either if they were at fault - but I've no real experience of this sort of fault - what's it likely to be - the pilot itself or the thermocouple - or maybe something else?
The room itself is well ventilated, so I don't think it's lack of oxygen or CO buildup. Pilot light burns with a blue flame (although I think it's a bit "fuzzy", but I don't know if it's meant to be like that!)
Loathe to even think about a new boiler as it's otherwise sound and has a functional (if rather ugly) gas fire attached, and is in a place where running a condensate drain would be virtually impossible (middle of house in an old fireplace!)
Any clues welcome!
Cheers,
Gordon
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With our very old gas boiler there were only two things that went wrong: (1) Thermocouple would fail every few years. (2) Pilot light jet would get blocked with crud every few years. The symptoms were similar to yours. Dust/dirt blocked the jet so it went out. Gas turned off. Dirt dropped back and cleared the jet. Lit the jet. Repeat as required. A good clean out of the pilot feed and jet cured this.
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Gordon Henderson explained :

Check the condition of the 'nose' of the thermocouple, see if it looks burnt. Check it is properly in the flame, check the draft shield is properly placed to stop the wind or drafts blowing it out and that the brass nut is not loose where it enters the gas valve.
If the flame looks poor, the jet could be sooted up.
--
Regards,
Harry (M1BYT) (L)
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Sounds like a bit of TLC might be all that's needed then. I'll have a look later today when it's cooled off a bit - it did stay in all night and heated the house this morning.
Thanks,
Gordon
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How long since it was serviced? Whilst many modern boilers actually work best by not taking them apart to service too often, open flued boilers such as your back boiler really do have to be checked over anually, because of the risk of CO emission if anything goes wrong. A sooted up pilot jet is a likely symptom of a boiler needing cleaning, because the soot may have resulted from incomplete combustion (and hence CO) already. Once such boilers start producing a tiny amount of soot, that in itself can lead to increased levels as it falls back into jets and burner air intakes, and the thing can rapidly run away, sooting itself up at a faster and faster rate. Just cleaning the pilot jet isn't really safe enough.
--
Andrew Gabriel
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Spring last year, so about 9 months.. I watched him do it - the thing was spotless inside bar some dust on the floor - and it looks fairly clean now, as far as I can tell, not having taken the fire off yet - I can see the pilot and gas burner pan from underneath.

Having observed first-hand the effects of CO posioning on a diving buddy, (admittedly accellerated due to the depth we were at) it's not an area I'm going to muck about in - if I'm at all unsure, I'm calling a man in!
But thinking now - I'm wondering if it's something as silly as wind direction - our log burners are making weird noises this week - there is a cowl on the chimney it's under, but it still felt a bit draughty under there when I looked this morning. We have all sorts of weird rooftops and chimney arrangements here... (And there is a whopping great hole in the room to the outside alleyway covered in grills for ventilation in the room it's in) Our old coal-fired rayburn cooker used to regularly blow-back before we replaced it and put a new whirly cowl on it's chimney...
Cheers,
Gordon
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