Problem solved. I mentioned that I had 1000 litres delivered when the tank only had 200 litres in it. The delivery into a nearly empty tank stirred up sediment that almost blocked the tank exit filter.
So the boiler would run for about 4 mins using the oil in the constant level valve with the...
It is an old boiler - 50 years - but has been working well.
This week, it starts OK and runs for nearly 3 minutes then shuts down. The spark cuts out after 80 seconds.
It is burning now with a yellowish flame with a ring of yellow flame around the bottom until it cuts out.
I can't find anything...