SuzJ - I have just asked that exact question of Good Housekeeping! I have a Gaggenau steam oven and they were brilliant and sent me this reply -
"I contacted the Gaggenau Home Economist on your behlaf and this is her reply:
The advice I would give the GH reader is to place the puddings in the solid tray provided on the second shelf level (counting from the bottom), set oven at 100°C and 100% humidity and cook for between 4-6 hours (the recipe we have for Christmas pudding in our recipe folder says that the longer the steaming, the darker the pudding will be, it will obviously also depend on the recipe the reader is using). "
Does that makes sense for your oven?
I cooked mine last night for about 5 hrs and they look good - proof will be on Xmas day I guess.