Do I still get normal holiday entitlement?

[Avearage]

So I'm due 5 months into my holiday entitlement, the normal rule for my company is half before Aug half after....... However then I would be on maternity and due back the moth of my new holiday starting.... Anyone know the rules etc???

I think you essentially 'accrue' holiday, so whilst you're entitled to a set amount a year, and can book as & when, it's assumed you will work the year.
With regards to maternity, you accrue holiday whilst on maternity leave, so when you come back you are entitled to all of the annual leave you've accrued as well as your new entitlement for the next year (assuming you're going over into the next annual leave year).
It will be up to you and your employer as to how you use your annual leave - many people tag it on to the end of their maternity leave, others get paid for it, or use it in the following year, or a combination.
So in your case your employer might be ok with you taking your annual leave that you will accrue whilst on maternity, before you go on maternity, or they may want to wait till you come back.

From my understanding, if you can't use up your annual entitlement before going off on maternity leave then you have to use it at the end. So you would effectively extend your maternity leave. Upon returning to work, you will be entitled to your full years leave again.