Paul,
Try Comet first. Like Jim said, it's easier. I thought you'd done that but reread after Jim's comment.
Last new mould I bought was an Arsenal mould (VERY nice mould), which supposedly comes "clean" but I had to use boiling water to get it clean. No big deal, but it goes to show. Last mould I bought wasn't new (sure looked like it) - a used LEE, and I know the guy who used it. He's no dummy, yet I had a time of it getting that one clean too, even though it looked perfect. Boiling water to the rescue again. That's a good rinse after the comet. I BOIL a whole tea kettle of water and pour it over the loose blocks lying in the bottom of the sink - the whole pot. You can't pick them up for a while after that.
If water beads on them after Dawn, Comet, Brake Cleaner - they get the boiling water.