Thanks all for taking the time to reply - I really appreciate it. He suggested us going to counselling so I think that is something I will explore before I make any final decisions. I just feel like I have to think about what is best for my DD's as well as what is best for me as an individual. TBH the idea of being single and starting over again with 2 kids really scares me, but I know that is not enough of a reason to stay.
He will still have to go to Perth but has said he'll go up and back in a day. He says that he has stopped all contact with her. I believe this for the moment but don't know how I'd ever trust that he wouldn't do something similar in the future if we had a rough patch in our marriage.
I also agree that it's not justifiable to use the lack of s*x in our marriage as an excuse, especially given that we were going through IVF in the lead up to this time and I was then pregnant and caring for a newborn whilst this relationship was going on.