I'm Australian (living in the UK) which means I can remain in a Schengen country/ies for 90 days in a 180 day period. I travel in and out of Schengen countries a lot and I was wondering how I can find out when my latest 180 day period started so that I can work out how much time I can spend in France for the next few months. I do not want to risk going over the 90 days.

Maybe ask the consulate or embassy in London of the country you first went to or spend the most time in. At a guess you would have to find the first date you entered the Schengen zone and add 180 days, then more multiples of 180 depending on how many lots of them you've been there for.