Well I lifted the burner up by almost 2 inches and gave it another go. It brought 7 gallons to a rolling boil in about 35 min. Still not great but much better. I'll use it that way for now and order a new burner later.
I think tomorrow I'll do some cutting and give it another go. Or maybe just push the support for the burner up a bit, I could bend it or use some shims under it.
thanks for the replies.
I had the same issue with mine. I cut the ring down so the bigger pot sits on the 3 supports. I also used the metal I cut to make some extra support for the bigger pot.. I'll go take a photo and post it for you..