Doing the subtraction sums for:
HEASMAN, Phillis, age 18, crime Larceny, tried at Old Bailey 13 Apr 1815, gives a birth year of 1797.
The IGI has a baptism of PHILLIS HEASMAN, 18 FEB 1798, Barcombe, Sussex, which might be her.
There is one problem however, I see in an earlier post of yours (Rootschat 2008) you have considered this but have given her parents as Thomas and Mary Heasman of Barcombe, but the IGI record states they were William and Ann.
Phillis Heasman, was one of 101 convicts transported on the Mary Ann in July 1815.
She was convicted at Middlesex Gaol Delivery for a term of 7 years on 05 April 1815
The Mary Ann departed: July, 1815, Destination: New South Wales
Source: Australian Joint Copying Project. Microfilm Roll 87, Class and Piece Number HO11/2, Page Number 216
Roy G