[acropolite (Phil)]

I wouldn't be relying on heat sink grease if that's the only method of electrical isolation. It is usual to have an electrical barrier in cases where electrical isolation is required often mica with the heatsink compound applied to both sides. You should be able to buy heat sink compound from Jaycar or Altronics, even DSE; it's common, there's even some listed on ebay, just google for heatsink compound. I have also seen Kapton Tape used as a heatsink barrier in inverters, although how effective it is, compared with Silicone heatsink compound I'm not sure.