Hi Joanna. Further to your renewed correspondence to that that appeared on this site during November of last year.
Instead of prison, those awaiting transportation were often put aboard redundant wooden ships (called Hulks) moored in Naval dockyards. Those from Horsham Assizes were frequently sent to Chatham or Portsmouth, but I could not tell you what happened to them if they subsequently died in either of those locations.
Another website also suggests that the family surname became or was formerly ATTREE which could make him, his parents and his siblings as follows:
Parents
His father Cephas (born in Brighton c 1716) and mother Mary (or Martha) nee Light (b Southampton c1725) married in Southampton on 16 June 1745.
Their Children
1. Cephas (Tree) Attree, b. CIR 1746, Brighton, Sussex,
2. Martha (Tree) Attree, b. CIR 1748, Brighton, Sussex,
The parents then relocated to the Hastings area and the settlement cirtificate you have found dated 1750 may explain why they were obliged to move.
Whilst in Hastings they added the following children to the family.
3. Anne (Tree) Attree, b. CIR 1751, Hastings, Sussex,
4. Mary (Tree) Attree, b. CIR 1752, Hastings, Sussex,
5. Betty (Tree) Attree, b. CIR 1754, Hastings, Sussex,
The IGI has your Cephas' first marriage to a Mary Kent (Brighton 1777) and a second marriage to the widow Hannah Taylor/Bridgeham in Brighton on 6 sept 1793.
The site above then lists his children as:-
1. Mary (Tree) Attree, b. CIR 1785, Brighton, Sussex,
2. James (Tree) Attree, b. 14 Dec 1787, Brighton, Sussex,
3. John (Tree) Attree, b. CIR 1789, Brighton, Sussex,
4. George (Tree) Attree, b. CIR 1792, Brighton, Sussex,
The grandparents (Stephen & Elizabeth) are shown as coming from Street, near Ditchling.
If this is your Cephas's line, perhaps you should conduct another search for him in the convict records, but under the alternative surname of Attree.
Roy G