I had to do another 'fix' this evening.
When I did last weeks repair I used some stainless steel Ben had laying around to create extra supports for the heatproof block in the top of the firebox. By about Wednesday last week I could hear something wasn't quite right, because after the burner shut down there was still a rumbling noise from the firebox. Popping the inspection hole while it shut down confirmed that flames were still present.
So tonight I pulled it apart again.
Not all Stainless Steel is created equal.
This stuff had burned through in a week, and the cause of the problem was that the bits had fallen into the burner. So this time it was a complete strip of the firebox, pulled the burner unit out, cleaned it up & checked it over before popping it all back together.
This time I hope all will be well. Until summer, when I have some preventive maintenance scheduled.