With
adultery you have two options:
1. You can
petition your ex for divorce on the grounds of
adultery with an un-named person. In this case your ex
will need to admit to the
adultery, or you will need undeniable evidence (eg a
child born from the adultery - just living togther is not enough).
2. You can also co-petition the girlfriend and have her named as party to
your divorce. However, she too will then need to admit to her part in the
adultery.
Alternatively, and much easier all
round, you can apply for divorce on the grounds of your ex's unreasonable
behaviour. You don't need him to agree to this and just need to show the
court why it is unreasonable for you to stay married. 4 or 5 bullet points
will do.
For example:
Ex moved out of the marital
home in XXX 2011, abandoning you and making the marriage unsustainable.
Ex is having an inappropriate personal relationship with another
woman who he now lives with.
Very good luck with everything x