I finish my herceptin (trastuzumab) treatment in 20th november 2012 and will have IVF to have my frozen embryos replaced. When would be the earliest I can do this?

Depends. The timing of pregnancy after cancer treatment varies considerably. Please discuss this with your oncologist. In most cases you will need a period of time, sometimes years without cancer before proceeding with pregnancy. In some cases it may be safer to avoid pregnancy and use a gestational surrogate. Again, these are issues that you should discuss with your oncologist.
Ask your oncologist . When it would be safe to try for a pregnancy. Good luck!