There was a comment on a different thread about wind disrupting the flame from a 3 ring burner.
I had all sorts of problems, so I had this knocked up out of galvanised steel. Last night my boiler brought 27 litres to a boil in about 30 minutes, so I'm now a happy chappy.
It's about 400mm high, and the only air vents are where I've cut it around the burner arm and ball valve.
It's very tight around the boiler, so virtually no heat will escape up the sides. The 2nd picture shows how the steel is folded back on itself to hold it together as it naturally wants to straighten itself.