Thanks for replying. A debris blockage was my first thought as well (we had this happen last time we had a delivery) although I would think that the blockage would be between the bleed nipple screw and the copper pipe oil fead into the burner (there is a braided metal pipe between the two which may contain the blockage if my thinking is correct but I can't unscrew it even using a wrench).
I am a complete novice with this sort of thing though so detailed idiot-proof instructions would be much appreciated!
The last time this happened, I disconnected the direct fead into the boiler which is a thin copper line with a wrench-turnable fitting to disconnect it. Last time, I disconnected that on the advice of the manufacturers and pressed the reset button. When I did that, I had a steady flow of oil come out albeit with a few bubbles which disappeared when I repeated the process. This time, only drops come out, perhaps two drops per second, and it doesn't seem to improve when I repeat.
The other problem is that I don't know what the filter looks like or where to find it. I've checked by the tank and (other than the tank isolation valve) there is no fitting on the pipe before it appears below the ground. The line reappears just before entering the house and the only fitting on the line looks like a copper 'bell'-like object that has a hard looping thin copper pipe coming out of the top that seems to end up connecting to the ingnition chamber (described as the 'flame' on my manual if memory serves (although I can't find the manual this time around)) so I'm not sure if this is a filter. Do you know where the filter may be on the burner (ie. before or after the bleed nipple screw for example) or what it looks like?
Thank you for the suggestion. Unfortunately, it hasn't worked. Is there anything else that you could suggest?
Just tried unscrewing the fire valve on the tank side. I definitely have a healthy flow of oil on the tank side of the fire valve!
Unfortunately, I don't think that I can safely undo the fire valve fully on the boiler side. It is very low off the floor and immediately above skirting board which makes it impossible to collect the oil that comes out. If I were to undo it, I would end up with the entire contents of the pipe from the tank isolator valve to the fire valve (roughly 10 metres) all over the wooden floor so I don't think I can do that.
The good news is that we now know that it is a blockage inside the house meaning that is much more easily accessible than a blockage in pipework outside under concrete. What I was thinking of doing is fully taking out the bleed nipple screw (which I only undid slightly before to get seepage) and blowing down it towards the boiler with the plastic lance I was using earlier.
The reason for that is that I suspect the blockage is somewhere between the bleed nipple and the copper pipe (perhaps in the flexible metal braided pipe or in the black fitting that is between the braided pipe and the copper one). When I loosened the bleed nipple screw slightly earlier, I got oil seeping out so I'm guessing the blockage is between the bleed nipple and the copper pipe. I don't know if there is a one-directional valve or something in the bleed nipple which would make this pointless though. Does this sound like a sensible thing to do?
I've just thought, before I try that, that it could be dangerous if I get any oil spillage inside the boiler (the bleed nipple being just above the burner). As the parts in the boiler heat up when the system is on and working, will any residual oil that I haven't managed to mop up reach combustion point?
Unfortunately, this did not resolve the issue. We had to call out a repair man to fix it in the end. It turned out to be related to the pressure in the system rather than a blockage.
Thank you for the help anyway.