The Challenger Amp presented a new opportunity to beef up my dummy load capacity, and dissapation of heat was the ambition. Home brewing 20 X non-inductive 1K resistors in parallel rated at 10watts each sadwhiched between two coper circles, submerged in a 2.5litre tin and filled with transformer oil seems to have done the trick! I used epoxy resin to seal the coax connector in the lid of the paint tin to prevent any leakage.
A bit scruffy but does work - SWR 1.0 160m thru to 1.3 at 10m, certainly handles 1200watts at short bursts. I havent yet run a load into it long enough for it to get hot.