As I understand it, you return to work on the same terms and conditions as those that stood when you left. What you are asking to do os to return under different terms and so your employer is entitled to negotiate different terms with you. You could extend your leave (unpaid) for a further six months.
The work and families act says that your employer has to seriously consider flexible hours - but is allowed to reject them for business reasons.
I do not think that you are being sexually discriminated against - as a father who took 6 months off and then requested part time work could be given the same decision.
You need to discuss the reasons with your employer, and try to negotiate with them regards to thos. Good luck