Hello

My partner and I had a fabulous time in Madrid in July and would like to get married there next July.

We want to have the ceremony in the Retiro by a celebrant. Does anyone know who to contact about using the Retiro?

Any contacts for english speaking celebrants in Madrid?

I am Australian and my partner is from the UK so we are researching our legal requirements too.

Is there anything else I should know?

Contact your your embassy in Madrid, they should be able to give you information on all the legal requirements and on english speaking officiants. If not, ask the american embassy, your legal requirements will be different but they have a list of english speaking officiants that you can have.
As far as the celebration in the retiro, I'm not sure, but ask the Ayuntamiento. It would be a good place to start.

Congratulations and Good luck!