At the moment UK law is SHIT at getting decent access for fathers if the mother decides otherwise
Have I missed something? Are you living in different countries? Your girls are old enough to make their own decisions as whether to see you or not - (the Law would be on your side). How CAN she stop them if your daughters want to?
As you don't feel able to 'grass' your ex - are you considering using it as a lever to gain easier access? OR are you suggesting someone else turn her in? lol - post the details, I'm sure someone will oblige.