william davis (alias brown)

I would like to trace the background of my 3x great grand father William Davis alias Brown. He was born about 1780 i assume in England (maybe War), i don't know where. He was sentenced to hang 1805for housebreaking in Horsham Sussex ENG and then it was changed to life transportation to Australia and arrived in Australia at Port Jackson on the Royal Admiral 2 in 1806. He married Mary Geer (another convict) in 1815 at Parramatta NSW. I have a little bit of info after that but as i can't find anything about his family and life in England i am hoping someone out there has some answers for me on his background.
Would really love some help on this if anyone is up for a challenge.
Rhonda