Success!! Yesterday afternoon I had another go with a blade roast (1.2kg), spuds & pumpkin. I waited about 50 mins for the beads to get hot & then stuck the empty oven on top for 10 mins to pre-heat. 12 beads on the bottom, 14 on top, 1 1/2 hrs later it was done beautifully (my wife doesn't like it when it still a little pink so I guess you could call it well done). The spuds and pumkin could have gone in a bit later than the meat but they tasted great anyway.
This is getting a bit addictive, today I think I might try a bread or a cake or something like that.