Looking for Family of Roston Johnson Son of Thomas Johnson Ann Mitchell of Brighton Plains Adelaide

Roston Johnson was my gt. gt. grandfather. His wife, Elizabeth Mewton, returned to England in the 1870's after his death. As Roston has been used as a family name ever since, I would really like to know its origins.

I have quite a few family papers, and written reminiscences, which I would be happy to share, and my sister and I would love to learn more about any extended family members, then and now, and to fill in as many gaps as possible.